The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 54

Cyclamen persic.u.m--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 tabac.u.m--crossed and self-fertilised flowers on the crossed and self-fertilised plants of the 1st generation yielded seeds as (by weight): 97.

Nicotiana tabac.u.m--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 tabac.u.m--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 Verbasc.u.m 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 Verbasc.u.m 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 Pa.s.siflora 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 Pa.s.siflora 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-ma.s.ses 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. proc.u.mbens 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 s.e.xual const.i.tution. 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 s.e.xual const.i.tution.

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.