THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS

The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom by Charles Darwin, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS

THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE PRODUCTION OF SEEDS

The present chapter is devoted to the Fertility of plants, as influenced by cross-fertilisation and self-fertilisation. The subject consists of two distinct branches; firstly, the relative productiveness or fertility of flowers crossed with pollen from a distinct plant and with their own pollen, as shown by the proportional number of capsules which they produce, together with the number of the contained seeds. Secondly, the degree of innate fertility or sterility of the seedlings raised from crossed and self-fertilised seeds; such seedlings being of the same age, grown under the same conditions, and fertilised in the same manner. These two branches of the subject correspond with the two which have to be considered by any one treating of hybrid plants; namely, in the first place the comparative productiveness of a species when fertilised with pollen from a distinct species and with its own pollen; and in the second place, the fertility of its hybrid offspring. These two classes of cases do not always run parallel; thus some plants, as Gartner has shown, can be crossed with great ease, but yield excessively sterile hybrids; while others are crossed with extreme difficulty, but yield fairly fertile hybrids.

The natural order to follow in this chapter would have been first to consider the effects on the fertility of the parent-plants of crossing them, and of fertilising them with their own pollen; but as we have discussed in the two last chapters the relative height, weight, and constitutional vigour of crossed and self-fertilised plants—that is, of plants raised from crossed and self-fertilised seeds—it will be convenient here first to consider their relative fertility. The cases observed by me are given in Table 9/D, in which plants of crossed and self-fertilised parentage were left to fertilise themselves, being either crossed by insects or spontaneously self-fertilised. It should be observed that the results cannot be considered as fully trustworthy, for the fertility of a plant is a most variable element, depending on its age, health, nature of the soil, amount of water given, and temperature to which it is exposed. The number of the capsules produced and the number of the contained seeds, ought to have been ascertained on a large number of crossed and self-fertilised plants of the same age and treated in every respect alike. In these two latter respects my observations may be trusted, but a sufficient number of capsules were counted only in a few instances. The fertility, or as it may perhaps better be called the productiveness, of a plant depends on the number of capsules produced, and on the number of seeds which these contain. But from various causes, chiefly from the want of time, I was often compelled to rely on the number of the capsules alone. Nevertheless, in the more interesting cases, the seeds were also counted or weighed. The average number of seeds per capsule is a more valuable criterion of fertility than the number of capsules produced. This latter circumstance depends partly on the size of the plant; and we know that crossed plants are generally taller and heavier than the self-fertilised; but the difference in this respect is rarely sufficient to account for the difference in the number of the capsules produced. It need hardly be added that in Table 9/D the same number of crossed and self-fertilised plants are always compared. Subject to the foregoing sources of doubt I will now give the table, in which the parentage of the plants experimented on, and the manner of determining their fertility are explained. Fuller details may be found in the previous part of this work, under the head of each species.

TABLE 9/D.—RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression, as 100 to x.

Ipomoea purpurea—first generation: seeds per capsule on crossed and self-fertilised plants, not growing much crowded, spontaneously self-fertilised under a net, in number: 99.

Ipomoea purpurea—seeds per capsule on crossed and self-fertilised plants from the same parents as in the last case, but growing much crowded, spontaneously self-fertilised under a net, in number: 93.

Ipomoea purpurea—productiveness of the same plants, as judged by the number of capsules produced, and average number of seeds per capsule: 45.

Ipomoea purpurea—third generation: seeds per capsule on crossed and self-fertilised plants, spontaneously self-fertilised under a net, in number: 94.

Ipomoea purpurea—productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 35.

Ipomoea purpurea—fifth generation: seeds per capsule on crossed and self-fertilised plants, left uncovered in the hothouse, and spontaneously fertilised: 89.

Ipomoea purpurea—ninth generation: number of capsules on crossed plants to those on self-fertilised plants, spontaneously self-fertilised under a net: 26.

Mimulus luteus—an equal number of capsules on plants descended from self-fertilised plants of the 8th generation crossed by a fresh stock, and on plants of the 9th self-fertilised generation, both sets having been left uncovered and spontaneously fertilised, contained seeds, by weight: 30.

Mimulus luteus—productiveness of the same plants, as judged by the number of capsules produced, and the average weight of seeds per capsule: 3.

Vandellia nummularifolia—seeds per capsule from cleistogene flowers on the crossed and self-fertilised plants, in number: 106.

Salvia coccinea—crossed plants, compared with self-fertilised plants, produced flowers, in number: 57.

Iberis umbellata—plants left uncovered in greenhouse; intercrossed plants of the 3rd generation, compared with self-fertilised plants of the 3rd generation, yielded seeds, in number: 75.

Iberis umbellata—plants from a cross between two varieties, compared with self-fertilised plants of the 3rd generation, yielded seeds, by weight : 75.

Papaver vagum—crossed and self-fertilised plants, left uncovered, produced capsules, in number: 99.

Eschscholtzia californica—Brazilian stock; plants left uncovered and cross-fertilised by bees; capsules on intercrossed plants of the 2nd generation, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 78.

Eschscholtzia californica—productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 89.

Eschscholtzia californica—plants left uncovered and cross-fertilised by bees; capsules on plants derived from intercrossed plants of the 2nd generation of the Brazilian stock crossed by English stock, compared with capsules on self-fertilised plants of 2nd generation, contained seeds, in number: 63.

Eschscholtzia californica—productiveness of the same plants, as judged by the number of capsules produced, and the average number of seeds per capsule: 40.

Reseda odorata—crossed and self-fertilised plants, left uncovered and cross-fertilised by bees; produced capsules in number (about): 100.

Viola tricolor—crossed and self-fertilised plants, left uncovered and cross-fertilised by bees, produced capsules in number: 10.

Delphinium consolida—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 56.

Viscaria oculata—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 77.

Dianthus caryophyllus—plants spontaneously self-fertilised under a net; capsules on intercrossed and self-fertilised plants of the 3rd generation contained seeds in number: 125.

Dianthus caryophyllus—plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by an intercrossed plant of the same stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 73.

Dianthus caryophyllus—plants left uncovered and cross-fertilised by insects: offspring from plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the 4th self-fertilised generation, produced seeds by weight: 33.

Tropaeolum minus—crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number: 64.

Limnanthes douglasii—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number (about): 100.

Lupinus luteus—crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, produced seeds in number (judged from only a few pods): 88.

Phaseolus multiflorus—crossed and self-fertilised plants, left uncovered in the greenhouse, produced seeds in number (about): 100.

Lathyrus odoratus—crossed and self-fertilised plants of the 2nd generation, left uncovered in the greenhouse, but certainly self-fertilised, produced pods in number: 91.

Clarkia elegans—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 60.

Nemophila insignis—crossed and self-fertilised plants, covered by a net and spontaneously self-fertilised in the greenhouse, produced capsules in number: 29.

Petunia violacea—left uncovered and cross-fertilised by insects: plants of the 5th intercrossed and self-fertilised generations produced seeds, as judged by the weight of an equal number of capsules: 86.

Petunia violacea—left uncovered as above: offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the 5th self-fertilised generation, produced seeds, as judged by the weight of an equal number of capsules: 46.

Cyclamen persicum—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 12.

Anagallis collina—crossed and self-fertilised plants, left uncovered in the greenhouse, produced capsules in number: 8.

Primula veris—left uncovered in open ground and cross-fertilised by insects: offspring from plants of the 3rd illegitimate generation crossed by a fresh stock, compared with plants of the 4th illegitimate and self-fertilised generation, produced capsules in number: 5.

Same plants in the following year: 3.5.

Primula veris—(equal-styled variety): left uncovered in open ground and cross-fertilised by insects: offspring from plants self-fertilised for two generations and then crossed by another variety, compared with plants of the 3rd self-fertilised generation, produced capsules in number: 15.

Primula veris—(equal-styled variety) same plants; average number of seeds per capsule: 71.

Primula veris—(equal-styled variety) productiveness of the same plants, as judged by the number of capsules produced and the average number of seeds per capsule: 11.

This table includes thirty-three cases relating to twenty-three species, and shows the degree of innate fertility of plants of crossed parentage in comparison with those of self-fertilised parentage; both lots being fertilised in the same manner. With several of the species, as with Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots were certainly cross-fertilised by insects, and so it probably was with several of the others; but in some of the species, as with Nemophila, and in some of the trials with Ipomoea and Dianthus, the plants were covered up, and both lots were spontaneously self-fertilised. This also was necessarily the case with the capsules produced by the cleistogene flowers of Vandellia.

The fertility of the crossed plants is represented in Table 9/D by 100, and that of the self-fertilised by the other figures. There are five cases in which the fertility of the self-fertilised plants is approximately equal to that of the crossed; nevertheless, in four of these cases the crossed plants were plainly taller, and in the fifth somewhat taller than the self-fertilised. But I should state that in some of these five cases the fertility of the two lots was not strictly ascertained, as the capsules were not actually counted, from appearing equal in number and from all apparently containing a full complement of seeds. In only two instances in the table, namely, with Vandellia and in the third generation of Dianthus, the capsules on the self-fertilised plants contained more seed than those on the crossed plants. With Dianthus the ratio between the number of seeds contained in the self-fertilised and crossed capsules was as 125 to 100; both sets of plants were left to fertilise themselves under a net; and it is almost certain that the greater fertility of the self-fertilised plants was here due merely to their having varied and become less strictly dichogamous, so as to mature their anthers and stigmas more nearly at the same time than is proper to the species. Excluding the seven cases now referred to, there remain twenty-six in which the crossed plants were manifestly much more fertile, sometimes to an extraordinary degree, than the self-fertilised with which they grew in competition. The most striking instances are those in which plants derived from a cross with a fresh stock are compared with plants of one of the later self-fertilised generations; yet there are some striking cases, as that of Viola, between the intercrossed plants of the same stock and the self-fertilised, even in the first generation. The results most to be trusted are those in which the productiveness of the plants was ascertained by the number of capsules produced by an equal number of plants, together with the actual or average number of seeds in each capsule. Of such cases there are twelve in the table, and the mean of their mean fertility is as 100 for the crossed plants, to 59 for the self-fertilised plants. The Primulaceae seem eminently liable to suffer in fertility from self-fertilisation.

The following short table, Table 9/E, includes four cases which have already been partly given in the last table.

TABLE 9/E.—INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK, COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION. FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS.

Column 1: Name of plant and feature observed.

Column 2: Plants from a cross with a fresh stock.

Column 3: Intercrossed plants of the same stock.

Column 4: Self-fertilised plants.

Mimulus luteus—the intercrossed plants are derived from a cross between two plants of the 8th self-fertilised generation. The self-fertilised plants belong to the 9th generation: 100 : 4 : 3.

Eschscholtzia californica—the intercrossed and self-fertilised plants belong to the 2nd generation: 100 : 45 : 40.

Dianthus caryophyllus—the intercrossed plants are derived from self-fertilised of the 3rd generation, crossed by intercrossed plants of the 3rd generation. The self-fertilised plants belong to the 4th generation: 100 : 45 : 33.

Petunia violacea—the intercrossed and self-fertilised plants belong to the 5th generation: 100 : 54 : 46.

NB.—In the above cases, excepting in that of Eschscholtzia, the plants derived from a cross with a fresh stock belong on the mother-side to the same stock with the intercrossed and self-fertilised plants, and to the corresponding generation.

These cases show us how greatly superior in innate fertility the seedlings from plants self-fertilised or intercrossed for several generations and then crossed by a fresh stock are, in comparison with the seedlings from plants of the old stock, either intercrossed or self-fertilised for the same number of generations. The three lots of plants in each case were left freely exposed to the visits of insects, and their flowers without doubt were cross-fertilised by them.

Table 9/E further shows us that in all four cases the intercrossed plants of the same stock still have a decided though small advantage in fertility over the self-fertilised plants.

With respect to the state of the reproductive organs in the self-fertilised plants of Tables 9/D and 9/E, only a few observations were made. In the seventh and eighth generation of Ipomoea, the anthers in the flowers of the self-fertilised plants were plainly smaller than those in the flowers of the intercrossed plants. The tendency to sterility in these same plants was also shown by the first-formed flowers, after they had been carefully fertilised, often dropping off, in the same manner as frequently occurs with hybrids. The flowers likewise tended to be monstrous. In the fourth generation of Petunia, the pollen produced by the self-fertilised and intercrossed plants was compared, and they were far more empty and shrivelled grains in the former.

RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE FIRST OR A SUCCEEDING GENERATION.

I will first treat of the parent-plants, which were raised from seeds purchased from nursery-gardens, or taken from plants growing in my garden, or growing wild, and surrounded in every case by many individuals of the same species. Plants thus circumstanced will commonly have been intercrossed by insects; so that the seedlings which were first experimented on will generally have been the product of a cross. Consequently any difference in the fertility of their flowers, when crossed and self-fertilised, will have been caused by the nature of the pollen employed; that is, whether it was taken from a distinct plant or from the same flower. The degrees of fertility shown in Table 9/F, were determined in each case by the average number of seeds per capsule, ascertained either by counting or weighing.

Another element ought properly to have been taken into account, namely, the proportion of flowers which yielded capsules when they were crossed and self-fertilised; and as crossed flowers generally produce a larger proportion of capsules, their superiority in fertility, if this element had been taken into account, would have been much more strongly marked than appears in Table 9/F. But had I thus acted, there would have been greater liability to error, as pollen applied to the stigma at the wrong time fails to produce any effect, independently of its greater or less potency. A good illustration of the great difference in the results which sometimes follows, if the number of capsules produced relatively to the number of flowers fertilised be included in the calculation, was afforded by Nolana prostrata. Thirty flowers on some plants of this species were crossed and produced twenty-seven capsules, each containing five seeds; thirty-two flowers on the same plants were self-fertilised and produced only six capsules, each containing five seeds. As the number of seeds per capsule is here the same, the fertility of the crossed and self-fertilised flowers is given in Table 9/F as equal, or as 100 to 100. But if the flowers which failed to produce capsules be included, the crossed flowers yielded on an average 4.50 seeds, whilst the self-fertilised flowers yielded only 0.94 seeds, so that their relative fertility would have been as 100 to 21. I should here state that it has been found convenient to reserve for separate discussion the cases of flowers which are usually quite sterile with their own pollen.

TABLE 9/f.—relative fertility of the flowers on the parent-plants used in my experiments, when fertilised with pollen from a distinct plant and with their own pollen. Fertility judged of by the average number of seeds per capsule. Fertility of crossed flowers taken as 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression 100 to x.

Ipomoea purpurea—crossed and self-fertilised flowers yielded seeds as (about): 100.

Mimulus luteus—crossed and self-fertilised flowers yielded seeds as (by weight): 79.

Linaria vulgaris—crossed and self-fertilised flowers yielded seeds as: 14.

Vandellia nummularifolia—crossed and self-fertilised flowers yielded seeds as: 67?

Gesneria pendulina—crossed and self-fertilised flowers yielded seeds as (by weight): 100.

Salvia coccinea—crossed and self-fertilised flowers yielded seeds as (about): 100.

Brassica oleracea—crossed and self-fertilised flowers yielded seeds as: 25.

Eschscholtzia californica—(English stock) crossed and self-fertilised flowers yielded seeds as (by weight): 71.

Eschscholtzia californica—(Brazilian stock grown in England) crossed and self-fertilised flowers yielded seeds (by weight) as (about): 15.

Delphinium consolida—crossed and self-fertilised flowers (self-fertilised capsules spontaneously produced, but result supported by other evidence) yielded seeds as: 59.

Viscaria oculata—crossed and self-fertilised flowers yielded seeds as (by weight): 38.

Viscaria oculata—crossed and self-fertilised flowers (crossed capsules compared on following year with spontaneously self-fertilised capsules) yielded seeds as : 58.

Dianthus caryophyllus—crossed and self-fertilised flowers yielded seeds as: 92.

Tropaeolum minus—crossed and self-fertilised flowers yielded seeds as: 92.

Tropaeolum tricolorum—crossed and self-fertilised flowers yielded seeds as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been introduced into this table, although seedlings were not raised from them; but of the Cuphea only six crossed and six self-fertilised capsules, and of the Tropaeolum only six crossed and eleven self-fertilised capsules, were compared. A larger proportion of the self-fertilised than of the crossed flowers of the Tropaeolum produced fruit.)

Limnanthes douglasii—crossed and self-fertilised flowers yielded seeds as (about): 100.

Sarothamnus scoparius—crossed and self-fertilised flowers yielded seeds as: 41.

Ononis minutissima—crossed and self-fertilised flowers yielded seeds as: 65.

Cuphea purpurea—crossed and self-fertilised flowers yielded seeds as: 113.

Passiflora gracilis—crossed and self-fertilised flowers yielded seeds as: 85.

Specularia speculum—crossed and self-fertilised flowers yielded seeds as: 72.

Lobelia fulgens—crossed and self-fertilised flowers yielded seeds as (about): 100.

Nemophila insignis—crossed and self-fertilised flowers yielded seeds as (by weight): 69.

Borago officinalis—crossed and self-fertilised flowers yielded seeds as: 60.

Nolana prostrata—crossed and self-fertilised flowers yielded seeds as: 100.

Petunia violacea—crossed and self-fertilised flowers yielded seeds as (by weight): 67.

Nicotiana tabacum—crossed and self-fertilised flowers yielded seeds as (by weight): 150.

Cyclamen persicum—crossed and self-fertilised flowers yielded seeds as: 38.

Anagallis collina—crossed and self-fertilised flowers yielded seeds as: 96.

Canna warscewiczi—crossed and self-fertilised flowers (on three generations of crossed and self-fertilised plants taken all together) yielded seeds as: 85.

Table 9/G gives the relative fertility of flowers on crossed plants again cross-fertilised, and of flowers on self-fertilised plants again self-fertilised, either in the first or in a later generation. Here two causes combine to diminish the fertility of the self-fertilised flowers; namely, the lesser efficacy of pollen from the same flower, and the innate lessened fertility of plants derived from self-fertilised seeds, which as we have seen in the previous Table 9/D is strongly marked. The fertility was determined in the same manner as in Table 9/F, that is, by the average number of seeds per capsule; and the same remarks as before, with respect to the different proportion of flowers which set capsules when they are cross-fertilised and self-fertilised, are here likewise applicable.

TABLE 9/G.—RELATIVE FERTILITY OF FLOWERS ON CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST OR SOME SUCCEEDING GENERATION; THE FORMER BEING AGAIN FERTILISED WITH POLLEN FROM A DISTINCT PLANT, AND THE LATTER AGAIN WITH THEIR OWN POLLEN. FERTILITY JUDGED OF BY THE AVERAGE NUMBER OF SEEDS PER CAPSULE. FERTILITY OF CROSSED FLOWERS TAKEN AS 100.

Column 1: Name of plant and feature observed.

Column 2: x, in the expression, 100 to x.

Ipomoea purpurea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the first generation yielded seeds as: 93.

Ipomoea purpurea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 3rd generation yielded seeds as: 94.

Ipomoea purpurea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as: 94.

Ipomoea purpurea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 5th generation yielded seeds as: 107.

Mimulus luteus—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 3rd generation yielded seeds as (by weight): 65.

Mimulus luteus—same plants of the 3rd generation treated in the same manner on the following year yielded seeds as (by weight): 34.

Mimulus luteus—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as (by weight): 40.

Viola tricolor—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 69.

Dianthus caryophyllus—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 65.

Dianthus caryophyllus—flowers on self-fertilised plants of the 3rd generation crossed by intercrossed plants, and other flowers again self-fertilised yielded seeds as: 97.

Dianthus caryophyllus—flowers on self-fertilised plants of the 3rd generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as: 127.

Lathytus odoratus—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as: 65.

Lobelia ramosa—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 60.

Petunia violacea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 68.

Petunia violacea—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 4th generation yielded seeds as (by weight): 72.

Petunia violacea—flowers on self-fertilised plants of the 4th generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as (by weight): 48.

Nicotiana tabacum—crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 97.

Nicotiana tabacum—flowers on self-fertilised plants of the 2nd generation crossed by intercrossed plants, and other flowers again self-fertilised yielded seeds as (by estimation): 110.

Nicotiana tabacum—flowers on self-fertilised plants of the 3rd generation crossed by a fresh stock, and other flowers again self-fertilised yielded seeds as (by estimation): 110.

Anagallis collina—flowers on red variety crossed by a blue variety, and other flowers on the red variety self-fertilised yielded seeds as: 48.

Canna warscewiczi—crossed and self-fertilised flowers on the crossed and self-fertilised plants of three generations taken together yielded seeds as: 85.

As both these tables relate to the fertility of flowers fertilised by pollen from another plant and by their own pollen, they may be considered together. The difference between them consists in the self-fertilised flowers in Table 9/G, being produced by self-fertilised parents, and the crossed flowers by crossed parents, which in the later generations had become somewhat closely inter-related, and had been subjected all the time to nearly the same conditions. These two tables include fifty cases relating to thirty-two species. The flowers on many other species were crossed and self-fertilised, but as only a few were thus treated, the results cannot be trusted, as far as fertility is concerned, and are not here given. Some other cases have been rejected, as the plants were in an unhealthy condition. If we look to the figures in the two tables expressing the ratios between the mean relative fertility of the crossed and self-fertilised flowers, we see that in a majority of cases (i.e., in thirty-five out of fifty) flowers fertilised by pollen from a distinct plant yield more, sometimes many more, seeds than flowers fertilised with their own pollen; and they commonly set a larger proportion of capsules. The degree of infertility of the self-fertilised flowers differs extremely in the different species, and even, as we shall see in the section on self-sterile plants, in the individuals of the same species, as well as under slightly changed conditions of life. Their fertility ranges from zero to fertility equalling that of the crossed flowers; and of this fact no explanation can be offered. There are fifteen cases in the two tables in which the number of seeds per capsule produced by the self-fertilised flowers equals or even exceeds that yielded by the crossed flowers. Some few of these cases are, I believe, accidental; that is, would not recur on a second trial. This was apparently the case with the plants of the fifth generation of Ipomoea, and in one of the experiments with Dianthus. Nicotiana offers the most anomalous case of any, as the self-fertilised flowers on the parent-plants, and on their descendants of the second and third generations, produced more seeds than did the crossed flowers; but we shall recur to this case when we treat of highly self-fertile varieties.

It might have been expected that the difference in fertility between the crossed and self-fertilised flowers would have been more strongly marked in Table 9/G, in which the plants of one set were derived from self-fertilised parents, than in Table 9/F, in which flowers on the parent-plants were self-fertilised for the first time. But this is not the case, as far as my scanty materials allow of any judgment. There is therefore no evidence at present, that the fertility of plants goes on diminishing in successive self-fertilised generations, although there is some rather weak evidence that this does occur with respect to their height or growth. But we should bear in mind that in the later generations the crossed plants had become more or less closely inter-related, and had been subjected all the time to nearly uniform conditions.

It is remarkable that there is no close correspondence, either in the parent-plants or in the successive generations, between the relative number of seeds produced by the crossed and self-fertilised flowers, and the relative powers of growth of the seedlings raised from such seeds. Thus, the crossed and self-fertilised flowers on the parent-plants of Ipomoea, Gesneria, Salvia, Limnanthes, Lobelia fulgens, and Nolana produced a nearly equal number of seeds, yet the plants raised from the crossed seeds exceeded considerably in height those raised from the self-fertilised seeds. The crossed flowers of Linaria and Viscaria yielded far more seeds than the self-fertilised flowers; and although the plants raised from the former were taller than those from the latter, they were not so in any corresponding degree. With Nicotiana the flowers fertilised with their own pollen were more productive than those crossed with pollen from a slightly different variety; yet the plants raised from the latter seeds were much taller, heavier, and more hardy than those raised from the self-fertilised seeds. On the other hand, the crossed seedlings of Eschscholtzia were neither taller nor heavier than the self-fertilised, although the crossed flowers were far more productive than the self-fertilised. But the best evidence of a want of correspondence between the number of seeds produced by crossed and self-fertilised flowers, and the vigour of the offspring raised from them, is afforded by the plants of the Brazilian and European stocks of Eschscholtzia, and likewise by certain individual plants of Reseda odorata; for it might have been expected that the seedlings from plants, the flowers of which were excessively self-sterile, would have profited in a greater degree by a cross, than the seedlings from plants which were moderately or fully self-fertile, and therefore apparently had no need to be crossed. But no such result followed in either case: for instance, the crossed and self-fertilised offspring from a highly self-fertile plant of Reseda odorata were in average height to each other as 100 to 82; whereas the similar offspring from an excessively self-sterile plant were as 100 to 92 in average height.

With respect to the innate fertility of the plants of crossed and self-fertilised parentage, given in the previous Table 9/D—that is, the number of seeds produced by both lots when their flowers were fertilised in the same manner,—nearly the same remarks are applicable, in reference to the absence of any close correspondence between their fertility and powers of growth, as in the case of the plants in the Tables 9/F and 9/G, just considered. Thus the crossed and self-fertilised plants of Ipomoea, Papaver, Reseda odorata, and Limnanthes were almost equally fertile, yet the former exceeded considerably in height the self-fertilised plants. On the other hand, the crossed and self-fertilised plants of Mimulus and Primula differed to an extreme degree in innate fertility, but by no means to a corresponding degree in height or vigour.

In all the cases of self-fertilised flowers included in Tables 9/E, 9/F, and 9/G, these were fertilised with their own pollen; but there is another form of self-fertilisation, namely, by pollen from other flowers on the same plant; but this latter method made no difference in comparison with the former in the number of seeds produced, or only a slight difference. Neither with Digitalis nor Dianthus were more seeds produced by the one method than by the other, to any trustworthy degree. With Ipomoea rather more seeds, in the proportion of 100 to 91, were produced from a crossed between flowers on the same plant than from strictly self-fertilised flowers; but I have reason to suspect that the result was accidental. With Origanum vulgare, however, a cross between flowers on plants propagated by stolons from the same stock certainly increased slightly their fertility. This likewise occurred, as we shall see in the next section, with Eschscholtzia, perhaps with Corydalis cava and Oncidium; but not so with Bignonia, Abutilon, Tabernaemontana, Senecio, and apparently Reseda odorata.

SELF-STERILE PLANTS.

The cases here to be described might have been introduced in Table 9/F, which gives the relative fertility of flowers fertilised with their own pollen, and with that from a distinct plant, but it has been found more convenient to keep them for separate discussion. The present cases must not be confounded with those to be given in the next chapter relatively to flowers which are sterile when insects are excluded; for such sterility depends not merely on the flowers being incapable of fertilisation with their own pollen, but on mechanical causes, by which their pollen is prevented from reaching the stigma, or on the pollen and stigma of the same flower being matured at different periods.

In the seventeenth chapter of my ‘Variation of Animals and Plants under Domestication’ I had occasion to enter fully on the present subject; and I will therefore here give only a brief abstract of the cases there described, but others must be added, as they have an important bearing on the present work. Kolreuter long ago described plants of Verbascum phoeniceum which during two years were sterile with their own pollen, but were easily fertilised by that of four other species; these plants however afterwards became more or less self-fertile in a strangely fluctuating manner. Mr. Scott also found that this species, as well as two of its varieties, were self-sterile, as did Gartner in the case of Verbascum nigrum. So it was, according to this latter author, with two plants of Lobelia fulgens, though the pollen and ovules of both were in an efficient state in relation to other species. Five species of Passiflora and certain individuals of a sixth species have been found sterile with their own pollen; but slight changes in their conditions, such as being grafted on another stock or a change of temperature, rendered them self-fertile. Flowers on a completely self-impotent plant of Passiflora alata fertilised with pollen from its own self-impotent seedlings were quite fertile. Mr. Scott, and afterwards Mr. Munro, found that some species of Oncidium and of Maxillaria cultivated in a hothouse in Edinburgh were quite sterile with their own pollen; and Fritz Muller found this to be the case with a large number of Orchidaceous genera growing in their native home of South Brazil. (9/2. ‘Botanische Zeitung’ 1868 page 114.) He also discovered that the pollen-masses of some orchids acted on their own stigmas like a poison; and it appears that Gartner formerly observed indications of this extraordinary fact in the case of some other plants.

Fritz Muller also states that a species of Bignonia and Tabernaemontana echinata are both sterile with their own pollen in their native country of Brazil. (9/3. Ibid 1868 page 626 and 1870 page 274.) Several Amaryllidaceous and Liliaceous plants are in the same predicament. Hildebrand observed with care Corydalis cava, and found it completely self-sterile (9/4. ‘Report of the International Horticultural Congress’ 1866.); but according to Caspary a few self-fertilised seeds are occasionally produced: Corydalis halleri is only slightly self-sterile, and C. intermedia not at all so. (9/5. ‘Botanische Zeitung’ June 27, 1873.) In another Fumariaceous genus, Hypecoum, Hildebrand observed that H. grandiflorum was highly self-sterile, whilst H. procumbens was fairly self-fertile. (9/6. ‘Jahrb. fur wiss. Botanik’ B. 7 page 464.) Thunbergia alata kept by me in a warm greenhouse was self-sterile early in the season, but at a later period produced many spontaneously self-fertilised fruits. So it was with Papaver vagum: another species, P. alpinum, was found by Professor H. Hoffmann to be quite self-sterile excepting on one occasion (9/7. ‘Zur Speciesfrage’ 1875 page 47.); whilst P. somniferum has been with me always completely self-sterile.

Eschscholtzia californica.

This species deserves a fuller consideration. A plant cultivated by Fritz Muller in South Brazil happened to flower a month before any of the others, and it did not produce a single capsule. This led him to make further observations during the next six generations, and he found that all his plants were completely sterile, unless they were crossed by insects or were artificially fertilised with pollen from a distinct plant, in which case they were completely fertile. (9/8. ‘Botanische Zeitung’ 1868 page 115 and 1869 page 223.) I was much surprised at this fact, as I had found that English plants, when covered by a net, set a considerable number of capsules; and that these contained seeds by weight, compared with those on plants intercrossed by the bees, as 71 to 100. Professor Hildebrand, however, found this species much more self-sterile in Germany than it was with me in England, for the capsules produced by self-fertilised flowers, compared with those from intercrossed flowers, contained seeds in the ratio of only 11 to 100. At my request Fritz Muller sent me from Brazil seeds of his self-sterile plants, from which I raised seedlings. Two of these were covered with a net, and one produced spontaneously only a single capsule containing no good seeds, but yet, when artificially fertilised with its own pollen, produced a few capsules. The other plant produced spontaneously under the net eight capsules, one of which contained no less than thirty seeds, and on an average about ten seeds per capsule. Eight flowers on these two plants were artificially self-fertilised, and produced seven capsules, containing on an average twelve seeds; eight other flowers were fertilised with pollen from a distinct plant of the Brazilian stock, and produced eight capsules, containing on an average about eighty seeds: this gives a ratio of 15 seeds for the self-fertilised capsules to 100 for the crossed capsules. Later in the season twelve other flowers on these two plants were artificially self-fertilised; but they yielded only two capsules, containing three and six seeds. It appears therefore that a lower temperature than that of Brazil favours the self-fertility of this plant, whilst a still lower temperature lessens it. As soon as the two plants which had been covered by the net were uncovered, they were visited by many bees,and it was interesting to observe how quickly they became, even the more sterile plant of the two, covered with young capsules. On the following year eight flowers on plants of the Brazilian stock of self-fertilised parentage (i.e., grandchildren of the plants which grew in Brazil) were again self-fertilised, and produced five capsules, containing on an average 27.4 seeds, with a maximum in one of forty-two seeds; so that their self-fertility had evidently increased greatly by being reared for two generations in England. On the whole we may conclude that plants of the Brazilian stock are much more self-fertile in this country than in Brazil, and less so than plants of the English stock in England; so that the plants of Brazilian parentage retained by inheritance some of their former sexual constitution. Conversely, seeds from English plants sent by me to Fritz Muller and grown in Brazil, were much more self-fertile than his plants which had been cultivated there for several generations; but he informs me that one of the plants of English parentage which did not flower the first year, and was thus exposed for two seasons to the climate of Brazil, proved quite self-sterile, like a Brazilian plant, showing how quickly the climate had acted on its sexual constitution.

Abutilon darwinii.

Seeds of this plant were sent me by Fritz Muller, who found it, as well as some other species of the same genus, quite sterile in its native home of South Brazil, unless fertilised with pollen from a distinct plant, either artificially or naturally by humming-birds. (9/9. ‘Jenaische Zeitschr. fur Naturwiss’ B. 7 1872 page 22 and 1873 page 441.) Several plants were raised from these seeds and kept in the hothouse. They produced flowers very early in the spring, and twenty of them were fertilised, some with pollen from the same flower, and some with pollen from other flowers on the same plants; but not a single capsule was thus produced, yet the stigmas twenty-seven hours after the application of the pollen were penetrated by the pollen-tubes. At the same time nineteen flowers were crossed with pollen from a distinct plant, and these produced thirteen capsules, all abounding with fine seeds. A greater number of capsules would have been produced by the cross, had not some of the nineteen flowers been on a plant which was afterwards proved to be from some unknown cause completely sterile with pollen of any kind. Thus far these plants behaved exactly like those in Brazil; but later in the season, in the latter part of May and in June, they began to produce under a net a few spontaneously self-fertilised capsules. As soon as this occurred, sixteen flowers were fertilised with their own pollen, and these produced five capsules, containing on an average 3.4 seeds. At the same time I selected by chance four capsules from the uncovered plants growing close by, the flowers of which I had seen visited by humble-bees, and these contained on an average 21.5 seeds; so that the seeds in the naturally intercrossed capsules to those in the self-fertilised capsules were as 100 to 16. The interesting point in this case is that these plants, which were unnaturally treated by being grown in pots in a hothouse, under another hemisphere, with a complete reversal of the seasons, were thus rendered slightly self-fertile, whereas they seem always to be completely self-sterile in their native home.

Senecio cruentus (greenhouse varieties, commonly called Cinerarias, probably derived from several fruticose or herbaceous species much intercrossed (9/10. I am much obliged to Mr. Moore and to Mr. Thiselton Dyer for giving me information with respect to the varieties on which I experimented. Mr. Moore believes that Senecio cruentas, tussilaginis, and perhaps heritieri, maderensis and populifolius have all been more or less blended together in our Cinerarias.))

Two purple-flowered varieties were placed under a net in the greenhouse, and four corymbs on each were repeatedly brushed with flowers from the other plant, so that their stigmas were well covered with each other’s pollen. Two of the eight corymbs thus treated produced very few seeds, but the other six produced on an average 41.3 seeds per corymb, and these germinated well. The stigmas on four other corymbs on both plants were well smeared with pollen from the flowers on their own corymbs; these eight corymbs produced altogether ten extremely poor seeds, which proved incapable of germinating. I examined many flowers on both plants, and found the stigmas spontaneously covered with pollen; but they produced not a single seed. These plants were afterwards left uncovered in the same house where many other Cinerarias were in flower; and the flowers were frequently visited by bees. They then produced plenty of seed, but one of the two plants less than the other, as this species shows some tendency to be dioecious.

The trial was repeated on another variety with white petals tipped with red. Many stigmas on two corymbs were covered with pollen from the foregoing purple variety, and these produced eleven and twenty-two seeds, which germinated well. A large number of the stigmas on several of the other corymbs were repeatedly smeared with pollen from their own corymb; but they yielded only five very poor seeds, which were incapable of germination. Therefore the above three plants belonging to two varieties, though growing vigorously and fertile with pollen from either of the other two plants, were utterly sterile with pollen from other flowers on the same plant.

Reseda odorata.

Having observed that certain individuals were self-sterile, I covered during the summer of 1868 seven plants under separate nets, and will call these plants A, B, C, D, E, F, G. They all appeared to be quite sterile with their own pollen, but fertile with that of any other plant.

Fourteen flowers on A were crossed with pollen from B or C, and produced thirteen fine capsules. Sixteen flowers were fertilised with pollen from other flowers on the same plant, but yielded not a single capsule.

Fourteen flowers on B were crossed with pollen from A, C or D, and all produced capsules; some of these were not very fine, yet they contained plenty of seeds. Eighteen flowers were fertilised with pollen from other flowers on the same plant, and produced not one capsule.

Ten flowers on C were crossed with pollen from A, B, D or E, and produced nine fine capsules. Nineteen flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.

Ten flowers on D were crossed with pollen from A, B, C or E, and produced nine fine capsules. Eighteen flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.

Seven flowers on E were crossed with pollen from A, C, or D, and all produced fine capsules. Eight flowers were fertilised with pollen from other flowers on the same plant, and produced no capsules.

On the plants F and G no flowers were crossed, but very many (number not recorded) were fertilised with pollen from other flowers on the same plants, and these did not produce a single capsule.

We thus see that fifty-five flowers on five of the above plants were reciprocally crossed in various ways; several flowers on each of these plants being fertilised with pollen from several of the other plants. These fifty-five flowers produced fifty-two capsules, almost all of which were of full size and contained an abundance of seeds. On the other hand, seventy-nine flowers (besides many others not recorded) were fertilised with pollen from other flowers on the same plants, and these did not produce a single capsule. In one case in which I examined the stigmas of the flowers fertilised with their own pollen, these were penetrated by the pollen-tubes, although such penetration produced no effect. Pollen falls generally, and I believe always, from the anthers on the stigmas of the same flower; yet only three out of the above seven protected plants produced spontaneously any capsules, and these it might have been thought must have been self-fertilised. There were altogether seven such capsules; but as they were all seated close to the artificially crossed flowers, I can hardly doubt that a few grains of foreign pollen had accidentally fallen on their stigmas. Besides the above seven plants, four others were kept covered under the SAME large net; and some of these produced here and there in the most capricious manner little groups of capsules; and this makes me believe that a bee, many of which settled on the outside of the net, being attracted by the odour, had on some one occasion found an entrance, and had intercrossed a few of the flowers.

In the spring of 1869 four plants raised from fresh seeds were carefully protected under separate nets; and now the result was widely different to what it was before. Three of these protected plants became actually loaded with capsules, especially during the early part of the summer; and this fact indicates that temperature produces some effect, but the experiment given in the following paragraph shows that the innate constitution of the plant is a far more important element. The fourth plant produced only a few capsules, many of them of small size; yet it was far more self-fertile than any of the seven plants tried during the previous year. The flowers on four small branches of this semi-self-sterile plant were smeared with pollen from one of the other plants, and they all produced fine capsules.

As I was much surprised at the difference in the results of the trials made during the two previous years, six fresh plants were protected by separate nets in the year 1870. Two of these proved almost completely self-sterile, for on carefully searching them I found only three small capsules, each containing either one or two seeds of small size, which, however, germinated. A few flowers on both these plants were reciprocally fertilised with each other’s pollen, and a few with pollen from one of the following self-fertile plants, and all these flowers produced fine capsules. The four other plants whilst still remaining protected beneath the nets presented a wonderful contrast (though one of them in a somewhat less degree than the others), for they became actually covered with spontaneously self-fertilised capsules, as numerous as, or very nearly so, and as fine as those on the unprotected plants growing near.

The above three spontaneously self-fertilised capsules produced by the two almost completely self-sterile plants, contained altogether five seeds; and from these I raised in the following year (1871) five plants, which were kept under separate nets. They grew to an extraordinarily large size, and on August 29th were examined. At first sight they appeared entirely destitute of capsules; but on carefully searching their many branches, two or three capsules were found on three of the plants, half-a-dozen on the fourth, and about eighteen on the fifth plant. But all these capsules were small, some being empty; the greater number contained only a single seed, and very rarely more than one. After this examination the nets were taken off, and the bees immediately carried pollen from one of these almost self-sterile plants to the other, for no other plants grew near. After a few weeks the ends of the branches on all five plants became covered with capsules, presenting a curious contrast with the lower and naked parts of the same long branches. These five plants therefore inherited almost exactly the same sexual constitution as their parents; and without doubt a self-sterile race of Mignonette could have been easily established.

Reseda lutea.

Plants of this species were raised from seeds gathered from a group of wild plants growing at no great distance from my garden. After casually observing that some of these plants were self-sterile, two plants taken by hazard were protected under separate nets. One of these soon became covered with spontaneously self-fertilised capsules, as numerous as those on the surrounding unprotected plants; so that it was evidently quite self-fertile. The other plant was partially self-sterile, producing very few capsules, many of which were of small size. When, however, this plant had grown tall, the uppermost branches became pressed against the net and grew crooked, and in this position the bees were able to suck the flowers through the meshes, and brought pollen to them from the neighbouring plants. These branches then became loaded with capsules; the other and lower branches remaining almost bare. The sexual constitution of this species is therefore similar to that of Reseda odorata.

CONCLUDING REMARKS ON SELF-STERILE PLANTS.

In order to favour as far as possible the self-fertilisation of some of the foregoing plants, all the flowers on Reseda odorata and some of those on the Abutilon were fertilised with pollen from other flowers on the same plant, instead of with their own pollen, and in the case of the Senecio with pollen from other flowers on the same corymb; but this made no difference in the result. Fritz Muller tried both kinds of self-fertilisation in the case of Bignonia, Tabernaemontana and Abutilon, likewise with no difference in the result. With Eschscholtzia, however, he found that pollen from other flowers on the same plant was a little more effective than pollen from the same flower. So did Hildebrand in Germany; as thirteen out of fourteen flowers of Eschscholtzia thus fertilised set capsules, these containing on an average 9.5 seeds; whereas only fourteen flowers out of twenty-one fertilised with their own pollen set capsules, these containing on an average 9.0 seeds. (9/11. ‘Pringsheim’s Jahrbuch fur wiss. Botanik’ 7 page 467.) Hildebrand found a trace of a similar difference with Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. ‘Variation under Domestication’ chapter 17 2nd edition volume 2 pages 113-115.)

In considering the several cases above given of complete or almost complete self-sterility, we are first struck with their wide distribution throughout the vegetable kingdom. Their number is not at present large, for they can be discovered only by protecting plants from insects and then fertilising them with pollen from another plant of the same species and with their own pollen; and the latter must be proved to be in an efficient state by other trials. Unless all this be done, it is impossible to know whether their self-sterility may not be due to the male or female reproductive organs, or to both, having been affected by changed conditions of life. As in the course of my experiments I have found three new cases, and as Fritz Muller has observed indications of several others, it is probable that they will hereafter be proved to be far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal in the United States quoted in ‘Gardeners’ Chronicle’ 1868 page 1286, states that Lilium auratum, Impatiens pallida and fulva, and Forsythia viridissima, cannot be fertilised with their own pollen.)

As with plants of the same species and parentage, some individuals are self-sterile and others self-fertile, of which fact Reseda odorata offers the most striking instances, it is not at all surprising that species of the same genus differ in this same manner. Thus Verbascum phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis are quite self-fertile, as I know by trial. There is the same difference between some of the species of Papaver, Corydalis, and of other genera. Nevertheless, the tendency to self-sterility certainly runs to a certain extent in groups, as we see in the genus Passiflora, and with the Vandeae amongst Orchids.

Self-sterility differs much in degree in different plants. In those extraordinary cases in which pollen from the same flower acts on the stigma like a poison, it is almost certain that the plants would never yield a single self-fertilised seed. Other plants, like Corydalis cava, occasionally, though very rarely, produce a few self-fertilised seeds. A large number of species, as may be seen in Table 9/F, are less fertile with their own pollen than with that from another plant; and lastly, some species are perfectly self-fertile. Even with the individuals of the same species, as just remarked, some are utterly self-sterile, others moderately so, and some perfectly self-fertile. The cause, whatever it may be, which renders many plants more or less sterile with their own pollen, that is, when they are self-fertilised, must be different, at least to a certain extent, from that which determines the difference in height, vigour, and fertility of the seedlings raised from self-fertilised and crossed seeds; for we have already seen that the two classes of cases do not by any means run parallel. This want of parallelism would be intelligible, if it could be shown that self-sterility depended solely on the incapacity of the pollen-tubes to penetrate the stigma of the same flower deeply enough to reach the ovules; whilst the greater or less vigorous growth of the seedlings no doubt depends on the nature of the contents of the pollen-grains and ovules. Now it is certain that with some plants the stigmatic secretion does not properly excite the pollen-grains, so that the tubes are not properly developed, if the pollen is taken from the same flower. This is the case according to Fritz Muller with Eschscholtzia, for he found that the pollen-tubes did not penetrate the stigma deeply; and with the Orchidaceous genus Notylia they failed altogether to penetrate it. (9/14. ‘Botanische Zeitung’ 1868 pages 114, 115.)

With dimorphic and trimorphic species, an illegitimate union between plants of the same form presents the closest analogy with self-fertilisation, whilst a legitimate union closely resembles cross-fertilisation; and here again the lessened fertility or complete sterility of an illegitimate union depends, at least in part, on the incapacity for interaction between the pollen-grains and stigma. Thus with Linum grandiflorum, as I have elsewhere shown, not more than two or three out of hundreds of pollen-grains, either of the long-styled or short-styled form, when placed on the stigma of their own form, emit their tubes, and these do not penetrate deeply; nor does the stigma itself change colour, as occurs when it is legitimately fertilised. (9/15. ‘Journal of the Linnean Society Botany’ volume 7 1863 pages 73-75.)

On the other hand the difference in innate fertility, as well as in growth between plants raised from crossed and self-fertilised seeds, and the difference in fertility and growth between the legitimate and illegitimate offspring of dimorphic and trimorphic plants, must depend on some incompatibility between the sexual elements contained within the pollen-grains and ovules, as it is through their union that new organisms are developed.

If we now turn to the more immediate cause of self-sterility, we clearly see that in most cases it is determined by the conditions to which the plants have been subjected. Thus Eschscholtzia is completely self-sterile in the hot climate of Brazil, but is perfectly fertile there with the pollen of any other individual. The offspring of Brazilian plants became in England in a single generation partially self-fertile, and still more so in the second generation. Conversely, the offspring of English plants, after growing for two seasons in Brazil, became in the first generation quite self-sterile. Again, Abutilon darwinii, which is self-sterile in its native home of Brazil, became moderately self-fertile in a single generation in an English hothouse. Some other plants are self-sterile during the early part of the year, and later in the season become self-fertile. Passiflora alata lost its self-sterility when grafted on another species. With Reseda, however, in which some individuals of the same parentage are self-sterile and others are self-fertile, we are forced in our ignorance to speak of the cause as due to spontaneous variability; but we should remember that the progenitors of these plants, either on the male or female side, may have been exposed to somewhat different conditions. The power of the environment thus to affect so readily and in so peculiar a manner the reproductive organs, is a fact which has many important bearings; and I have therefore thought the foregoing details worth giving. For instance, the sterility of many animals and plants under changed conditions of life, such as confinement, evidently comes within the same general principle of the sexual system being easily affected by the environment. It has already been proved, that a cross between plants which have been self-fertilised or intercrossed during several generations, having been kept all the time under closely similar conditions, does not benefit the offspring; and on the other hand, that a cross between plants that have been subjected to different conditions benefits the offspring to an extraordinary degree. We may therefore conclude that some degree of differentiation in the sexual system is necessary for the full fertility of the parent-plants and for the full vigour of their offspring. It seems also probable that with those plants which are capable of complete self-fertilisation, the male and female elements and organs already differ to an extent sufficient to excite their mutual interaction; but that when such plants are taken to another country, and become in consequence self-sterile, their sexual elements and organs are so acted on as to be rendered too uniform for such interaction, like those of a self-fertilised plant long cultivated under the same conditions. Conversely, we may further infer that plants which are self-sterile in their native country, but become self-fertile under changed conditions, have their sexual elements so acted on, that they become sufficiently differentiated for mutual interaction.

We know that self-fertilised seedlings are inferior in many respects to those from a cross; and as with plants in a state of nature pollen from the same flower can hardly fail to be often left by insects or by the wind on the stigma, it seems at first sight highly probable that self-sterility has been gradually acquired through natural selection in order to prevent self-fertilisation. It is no valid objection to this belief that the structure of some flowers, and the dichogamous condition of many others, suffice to prevent the pollen reaching the stigma of the same flower; for we should remember that with most species many flowers expand at the same time, and that pollen from the same plant is equally injurious or nearly so as that from the same flower. Nevertheless, the belief that self-sterility is a quality which has been gradually acquired for the special purpose of preventing self-fertilisation must, I believe, be rejected. In the first place, there is no close correspondence in degree between the sterility of the parent-plants when self-fertilised, and the extent to which their offspring suffer in vigour by this process; and some such correspondence might have been expected if self-sterility had been acquired on account of the injury caused by self-fertilisation. The fact of individuals of the same parentage differing greatly in their degree of self-sterility is likewise opposed to such a belief; unless, indeed, we suppose that certain individuals have been rendered self-sterile to favour intercrossing, whilst other individuals have been rendered self-fertile to ensure the propagation of the species. The fact of self-sterile individuals appearing only occasionally, as in the case of Lobelia, does not countenance this latter view. But the strongest argument against the belief that self-sterility has been acquired to prevent self-fertilisation, is the immediate and powerful effect of changed conditions in either causing or in removing self-sterility. We are not therefore justified in admitting that this peculiar state of the reproductive system has been gradually acquired through natural selection; but we must look at it as an incidental result, dependent on the conditions to which the plants have been subjected, like the ordinary sterility caused in the case of animals by confinement, and in the case of plants by too much manure, heat, etc. I do not, however, wish to maintain that self-sterility may not sometimes be of service to a plant in preventing self-fertilisation; but there are so many other means by which this result might be prevented or rendered difficult, including as we shall see in the next chapter the prepotency of pollen from a distinct individual over a plant’s own pollen, that self-sterility seems an almost superfluous acquirement for this purpose.

Finally, the most interesting point in regard to self-sterile plants is the evidence which they afford of the advantage, or rather of the necessity, of some degree or kind of differentiation in the sexual elements, in order that they should unite and give birth to a new being. It was ascertained that the five plants of Reseda odorata which were selected by chance, could be perfectly fertilised by pollen taken from any one of them, but not by their own pollen; and a few additional trials were made with some other individuals, which I have not thought worth recording. So again, Hildebrand and Fritz Muller frequently speak of self-sterile plants being fertile with the pollen of any other individual; and if there had been any exceptions to the rule, these could hardly have escaped their observation and my own. We may therefore confidently assert that a self-sterile plant can be fertilised by the pollen of any one out of a thousand or ten thousand individuals of the same species, but not by its own. Now it is obviously impossible that the sexual organs and elements of every individual can have been specialised with respect to every other individual. But there is no difficulty in believing that the sexual elements of each differ slightly in the same diversified manner as do their external characters; and it has often been remarked that no two individuals are absolutely alike. Therefore we can hardly avoid the conclusion, that differences of an analogous and indefinite nature in the reproductive system are sufficient to excite the mutual action of the sexual elements, and that unless there be such differentiation fertility fails.

THE APPEARANCE OF HIGHLY SELF-FERTILE VARIETIES.

We have just seen that the degree to which flowers are capable of being fertilised with their own pollen differs much, both with the species of the same genus, and sometimes with the individuals of the same species. Some allied cases of the appearance of varieties which, when self-fertilised, yield more seed and produce offspring growing taller than their self-fertilised parents, or than the intercrossed plants of the corresponding generation, will now be considered.

Firstly, in the third and fourth generations of Mimulus luteus, a tall variety, often alluded to, having large white flowers blotched with crimson, appeared amongst both the intercrossed and self-fertilised plants. It prevailed in all the later self-fertilised generations to the exclusion of every other variety, and transmitted its characters faithfully, but disappeared from the intercrossed plants, owing no doubt to their characters being repeatedly blended by crossing. The self-fertilised plants belonging to this variety were not only taller, but more fertile than the intercrossed plants; though these latter in the earlier generations were much taller and more fertile than the self-fertilised plants. Thus in the fifth generation the self-fertilised plants were to the intercrossed in height as 126 to 100. In the sixth generation they were likewise much taller and finer plants, but were not actually measured; they produced capsules compared with those on the intercrossed plants, in number, as 147 to 100; and the self-fertilised capsules contained a greater number of seeds. In the seventh generation the self-fertilised plants were to the crossed in height as 137 to 100; and twenty flowers on these self-fertilised plants fertilised with their own pollen yielded nineteen very fine capsules,—a degree of self-sterility which I have not seen equalled in any other case. This variety seems to have become specially adapted to profit in every way by self-fertilisation, although this process was so injurious to the parent-plants during the first four generations. It should however be remembered that seedlings raised from this variety, when crossed by a fresh stock, were wonderfully superior in height and fertility to the self-fertilised plants of the corresponding generation.

Secondly, in the sixth self-fertilised generation of Ipomoea a single plant named the Hero appeared, which exceeded by a little in height its intercrossed opponent,—a case which had not occurred in any previous generation. Hero transmitted the peculiar colour of its flowers, as well as its increased tallness and a high degree of self-fertility, to its children, grandchildren, and great-grandchildren. The self-fertilised children of Hero were in height to other self-fertilised plants of the same stock as 100 to 85. Ten self-fertilised capsules produced by the grandchildren contained on an average 5.2 seeds; and this is a higher average than was yielded in any other generation by the capsules of self-fertilised flowers. The great-grandchildren of Hero derived from a cross with a fresh stock were so unhealthy, from having been grown at an unfavourable season, that their average height in comparison with that of the self-fertilised plants cannot be judged of with any safety; but it did not appear that they had profited even by a cross of this kind.

Thirdly, the plants of Nicotiana on which I experimented appear to come under the present class of cases; for they varied in their sexual constitution and were more or less highly self-fertile. They were probably the offspring of plants which had been spontaneously self-fertilised under glass for several generations in this country. The flowers on the parent-plants which were first fertilised by me with their own pollen yielded half again as many seeds as did those which were crossed; and the seedlings raised from these self-fertilised seeds exceeded in height those raised from the crossed seeds to an extraordinary degree. In the second and third generations, although the self-fertilised plants did not exceed the crossed in height, yet their self-fertilised flowers yielded on two occasions considerably more seeds than the crossed flowers, even than those which were crossed with pollen from a distinct stock or variety.

Lastly, as certain individual plants of Reseda odorata and lutea are incomparably more self-fertile than other individuals, the former might be included under the present heading of the appearance of new and highly self-fertile varieties. But in this case we should have to look at these two species as normally self-sterile; and this, judging by my experience, appears to be the correct view.

We may therefore conclude from the facts now given, that varieties sometimes arise which when self-fertilised possess an increased power of producing seeds and of growing to a greater height, than the intercrossed or self-fertilised plants of the corresponding generation—all the plants being of course subjected to the same conditions. The appearance of such varieties is interesting, as it bears on the existence under nature of plants which regularly fertilise themselves, such as Ophrys apifera and a few other orchids, or as Leersia oryzoides, which produces an abundance of cleistogene flowers, but most rarely flowers capable of cross-fertilisation.

Some observations made on other plants lead me to suspect that self-fertilisation is in some respects beneficial; although the benefit thus derived is as a rule very small compared with that from a cross with a distinct plant. Thus we have seen in the last chapter that seedlings of Ipomoea and Mimulus raised from flowers fertilised with their own pollen, which is the strictest possible form of self-fertilisation, were superior in height, weight, and in early flowering to the seedlings raised from flowers crossed with pollen from other flowers on the same plant; and this superiority apparently was too strongly marked to be accidental. Again, the cultivated varieties of the common pea are highly self-fertile, although they have been self-fertilised for many generations; and they exceeded in height seedlings from a cross between two plants belonging to the same variety in the ratio of 115 to 100; but then only four pairs of plants were measured and compared. The self-fertility of Primula veris increased after several generations of illegitimate fertilisation, which is a process closely analogous to self-fertilisation, but only as long as the plants were cultivated under the same favourable conditions. I have also elsewhere shown that with Primula veris and sinensis, equal-styled varieties occasionally appear which possess the sexual organs of the two forms combined in the same flower. (9/16. ‘Journal of the Linnean Society Botany’ volume 10 1867 pages 417, 419.) Consequently they fertilise themselves in a legitimate manner and are highly self-fertile; but the remarkable fact is that they are rather more fertile than ordinary plants of the same species legitimately fertilised by pollen from a distinct individual. Formerly it appeared to me probable, that the increased fertility of these dimorphic plants might be accounted for by the stigma lying so close to the anthers that it was impregnated at the most favourable age and time of the day; but this explanation is not applicable to the above given cases, in which the flowers were artificially fertilised with their own pollen.

Considering the facts now adduced, including the appearance of those varieties which are more fertile and taller than their parents and than the intercrossed plants of the corresponding generation, it is difficult to avoid the suspicion that self-fertilisation is in some respects advantageous; though if this be really the case, any such advantage is as a rule quite insignificant compared with that from a cross with a distinct plant, and especially with one of a fresh stock. Should this suspicion be hereafter verified, it would throw light, as we shall see in the next chapter, on the existence of plants bearing small and inconspicuous flowers which are rarely visited by insects, and therefore are rarely intercrossed.

RELATIVE WEIGHT AND PERIOD OF GERMINATION OF SEEDS FROM CROSSED AND SELF-FERTILISED FLOWERS.

An equal number of seeds from flowers fertilised with pollen from another plant, and from flowers fertilised with their own pollen, were weighed, but only in sixteen cases. Their relative weights are given in the following list; that of the seeds from the crossed flowers being taken as 100.

Column 1: Name of Plant.

Column 2: x, in the expression, 100 to x.

Ipomoea purpurea (parent plants): 127. Ipomoea purpurea (third generation): 87. Salvia coccinea: 100. Brassica oleracea: 103. Iberis umbellata (second generation): 136. Delphinium consolida: 45. Hibiscus africanus: 105. Tropaeolum minus: 115. Lathyrus odoratus (about): 100. Sarothamnus scoparius: 88. Specularia speculum: 86. Nemophila insignis: 105. Borago officinalis: 111. Cyclamen persicum (about): 50. Fagopyrum esculentum: 82. Canna warscewiczi (3 generations): 102.

It is remarkable that in ten out of these sixteen cases the self-fertilised seeds were either superior or equal to the crossed in weight; nevertheless, in six out of the ten cases (namely, with Ipomoea, Salvia, Brassica, Tropaeolum, Lathyrus, and Nemophila) the plants raised from these self-fertilised seeds were very inferior in height and in other respects to those raised from the crossed seeds. The superiority in weight of the self-fertilised seeds in at least six out of the ten cases, namely, with Brassica, Hibiscus, Tropaeolum, Nemophila, Borago, and Canna, may be accounted for in part by the self-fertilised capsules containing fewer seeds; for when a capsule contains only a few seeds, these will be apt to be better nourished, so as to be heavier, than when many are contained in the same capsule. It should, however, be observed that in some of the above cases, in which the crossed seeds were the heaviest, as with Sarothamnus and Cyclamen, the crossed capsules contained a larger number of seeds. Whatever may be the explanation of the self-fertilised seeds being often the heaviest, it is remarkable in the case of Brassica, Tropaeolum, Nemophila, and of the first generation of Ipomoea, that the seedlings raised from them were inferior in height and in other respects to the seedlings raised from the crossed seeds. This fact shows how superior in constitutional vigour the crossed seedlings must have been, for it cannot be doubted that heavy and fine seeds tend to yield the finest plants. Mr. Galton has shown that this holds good with Lathyrus odoratus; as has Mr. A.J. Wilson with the Swedish turnip, Brassica campestris ruta baga. Mr. Wilson separated the largest and smallest seeds of this latter plant, the ratio between the weights of the two lots being as 100 to 59, and he found that the seedlings “from the larger seeds took the lead and maintained their superiority to the last, both in height and thickness of stem.” (9/17. ‘Gardeners’ Chronicle’ 1867 page 107. Loiseleur-Deslongchamp ‘Les Cereales’ 1842 pages 208-219, was led by his observations to the extraordinary conclusion that the smaller grains of cereals produce as fine plants as the large. This conclusion is, however, contradicted by Major Hallet’s great success in improving wheat by the selection of the finest grains. It is possible, however, that man, by long-continued selection, may have given to the grains of the cereals a greater amount of starch or other matter, than the seedlings can utilise for their growth. There can be little doubt, as Humboldt long ago remarked, that the grains of cereals have been rendered attractive to birds in a degree which is highly injurious to the species.) Nor can this difference in the growth of the seedling turnips be attributed to the heavier seeds having been of crossed, and the lighter of self-fertilised origin, for it is known that plants belonging to this genus are habitually intercrossed by insects.

With respect to the relative period of germination of crossed and self-fertilised seeds, a record was kept in only twenty-one cases; and the results are very perplexing. Neglecting one case in which the two lots germinated simultaneously, in ten cases or exactly one-half many of the self-fertilised seeds germinated before the crossed, and in the other half many of the crossed before the self-fertilised. In four out of these twenty cases, seeds derived from a cross with a fresh stock were compared with self-fertilised seeds from one of the later self-fertilised generations; and here again in half the cases the crossed seeds, and in the other half the self-fertilised seeds, germinated first. Yet the seedlings of Mimulus raised from such self-fertilised seeds were inferior in all respects to the crossed seedlings, and in the case of Eschscholtzia they were inferior in fertility. Unfortunately the relative weight of the two lots of seeds was ascertained in only a few instances in which their germination was observed; but with Ipomoea and I believe with some of the other species, the relative lightness of the self-fertilised seeds apparently determined their early germination, probably owing to the smaller mass being favourable to the more rapid completion of the chemical and morphological changes necessary for germination. On the other hand, Mr. Galton gave me seeds (no doubt all self-fertilised) of Lathyrus odoratus, which were divided into two lots of heavier and lighter seeds; and several of the former germinated first. It is evident that many more observations are necessary before anything can be decided with respect to the relative period of germination of crossed and self-fertilised seeds.

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