The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 48

Only four crossed and four self-fertilised plants were raised and measured, and the former were to the latter in height as 100 to 102. So small a number of measurements ought never to be trusted; and in the present instance the advantage of the self-fertilised over the crossed plants depended almost entirely on one of the self-fertilised plants having grown to an unusual height. All four crossed plants flowered before their self-fertilised opponents. The cross-fertilised flowers on the parent-plants in comparison with the self-fertilised flowers yielded seeds in the proportion of 100 to 60. So that here again we may draw the same conclusion as in the two last cases.

10. Pa.s.siflora gracilis.

Only two crossed and two self-fertilised plants were raised; and the former were to the latter in height as 100 to 104. On the other hand, fruits from the cross-fertilised flowers on the parent-plants contained seeds in number, compared with those from the self-fertilised flowers, in the proportion of 100 to 85.

11. Phaseolus multiflorus.

The five crossed plants were to the five self-fertilised in height as 100 to 96. Although the crossed plants were thus only four per cent taller than the self-fertilised, they flowered in both pots before them.

It is therefore probable that they had some real advantage over the self-fertilised plants.

12. Adonis aestivalis.

The four crossed plants were almost exactly equal in height to the four self-fertilised plants, but as so few plants were measured, and as these were all "miserably unhealthy," nothing can be inferred with safety with respect to their relative heights.

13. Bartonia aurea.

The eight crossed plants were to the eight self-fertilised in height as 100 to 107. This number of plants, considering the care with which they were raised and compared, ought to have given a trustworthy result. But from some unknown cause they grew very unequally, and they became so unhealthy that only three of the crossed and three of the self-fertilised plants set any seeds, and these few in number. Under these circ.u.mstances the mean height of neither lot can be trusted, and the experiment is valueless. The cross-fertilised flowers on the parent-plants yielded rather more seeds than the self-fertilised flowers.

14. Thunbergia alata.

The six crossed plants were to the six self-fertilised in height as 100 to 108. Here the self-fertilised plants seem to have a decided advantage; but both lots grew unequally, some of the plants in both being more than twice as tall as others. The parent-plants also were in an odd semi-sterile condition. Under these circ.u.mstances the superiority of the self-fertilised plants cannot be fully trusted.

15. Nolana prostrata.

The five crossed plants were to the five self-fertilised in height as 100 to 105; so that the latter seem here to have a small but decided advantage. On the other hand, the flowers on the parent-plants which were cross-fertilised produced very many more capsules than the self-fertilised flowers, in the ratio of 100 to 21; and the seeds which the former contained were heavier than an equal number from the self-fertilised capsules in the ratio of 100 to 82.

16. Hibiscus africa.n.u.s.

Only four pairs were raised, and the crossed were to the self-fertilised in height as 100 to 109. Excepting that too few plants were measured, I know of nothing else to cause distrust in the result. The cross-fertilised flowers on the parent-plants were, on the other hand, rather more productive than the self-fertilised flowers.

17. Apium petroselinum.

A few plants (number not recorded) derived from flowers believed to have been crossed by insects and a few self-fertilised plants were grown on the opposite sides of four pots. They attained to a nearly equal height, the crossed having a very slight advantage.

18. Vandellia nummularifolia.

Twenty crossed plants raised from the seeds of perfect flowers were to twenty self-fertilised plants, likewise raised from the seeds of perfect flowers, in height as 100 to 99. The experiment was repeated, with the sole difference that the plants were allowed to grow more crowded; and now the twenty-four tallest of the crossed plants were to the twenty-four tallest self-fertilised plants in height as 100 to 94, and in weight as 100 to 97. Moreover, a larger number of the crossed than of the self-fertilised plants grew to a moderate height. The above-mentioned twenty crossed plants were also grown in compet.i.tion with twenty self-fertilised plants raised from the closed or cleistogene flowers, and their heights were as 100 to 94. Therefore had it not been for the first trial, in which the crossed plants were to the self-fertilised in height only as 100 to 99, this species might have been cla.s.sed with those in which the crossed plants exceed the self-fertilised by above five per cent. On the other hand, the crossed plants in the second trial bore fewer capsules; and these contained fewer seeds, than did the self-fertilised plants, all the capsules having been produced by cleistogene flowers. The whole case therefore must be left doubtful.

19. Pisum sativum (common pea).

Four-plants derived from a cross between individuals of the same variety were in height to four self-fertilised plants belonging to the same variety as 100 to 115. Although this cross did no good, we have seen under Table 7/C that a cross between distinct varieties adds greatly to the height and vigour of the offspring; and it was there explained that the fact of a cross between the individuals of the same variety not being beneficial, is almost certainly due to their having been self-fertilised for many generations, and in each generation grown under nearly similar conditions.

20, 21, 22. Canna warscewiczi.

Plants belonging to three generations were observed, and in all of three the crossed were approximately equal to the self-fertilised; the average height of the thirty-four crossed plants being to that of the same number of self-fertilised plants as 100 to 101. Therefore the crossed plants had no advantage over the self-fertilised; and it is probable that the same explanation here holds good as in the case of Pisum sativum; for the flowers of this Canna are perfectly self-fertile, and were never seen to be visited by insects in the hothouse, so as to be crossed by them. This plant, moreover, has been cultivated under gla.s.s for several generations in pots, and therefore under nearly uniform conditions. The capsules produced by the cross-fertilised flowers on the above thirty-four crossed plants contained more seeds than did the capsules produced by the self-fertilised flowers on the self-fertilised plants, in the proportion of 100 to 85; so that in this respect crossing was beneficial.

23. Primula sinensis.

The offspring of plants, some of which were legitimately and others illegitimately fertilised with pollen from a distinct plant, were almost exactly of the same height as the offspring of self-fertilised plants; but the former with rare exceptions flowered before the latter. I have shown in my paper on dimorphic plants that this species is commonly raised in England from self-fertilised seed, and the plants from having been cultivated in pots have been subjected to nearly uniform conditions. Moreover, many of them are now varying and changing their character, so as to become in a greater or less degree equal-styled, and in consequence highly self-fertile. Therefore I believe that the cause of the crossed plants not exceeding in height the self-fertilised is the same as in the two previous cases of Pisum sativum and Canna.

24, 25, 26. Nicotiana tabac.u.m.

Four sets of measurements were made; in one, the self-fertilised plants greatly exceeded in height the crossed, in two others they were approximately equal to the crossed, and in the fourth were beaten by them; but this latter case does not here concern us. The individual plants differ in const.i.tution, so that the descendants of some profit by their parents having been intercrossed, whilst others do not. Taking all three generations together, the twenty-seven crossed plants were in height to the twenty-seven self-fertilised plants as 100 to 96. This excess of height in the crossed plants, is so small compared with that displayed by the offspring from the same mother-plants when crossed by a slightly different variety, that we may suspect (as explained under Table 7/C) that most of the individuals belonging to the variety which served as the mother-plants in my experiments, had acquired a nearly similar const.i.tution, so as not to profit by being mutually intercrossed.]

Reviewing these twenty-six cases, in which the crossed plants either do not exceed the self-fertilised by above five per cent in height, or are inferior to them, we may conclude that much the greater number of the cases do not form real exceptions to the rule,--that a cross between two plants, unless these have been self-fertilised and exposed to nearly the same conditions for many generations, gives a great advantage of some kind to the offspring. Of the twenty-six cases, at least two, namely, those of Adonis and Bartonia, may be wholly excluded, as the trials were worthless from the extreme unhealthiness of the plants. Inn twelve other cases (three trials with Eschscholtzia here included) the crossed plants either were superior in height to the self-fertilised in all the other generations excepting the one in question, or they showed their superiority in some different manner, as in weight, fertility, or in flowering first; or again, the cross-fertilised flowers on the mother-plant were much more productive of seed than the self-fertilised.

Deducting these fourteen cases, there remain twelve in which the crossed plants show no well-marked advantage over the self-fertilised. On the other hand, we have seen that there are fifty-seven cases in which the crossed plants exceed the self-fertilised in height by at least five per cent, and generally in a much higher degree. But even in the twelve cases just referred to, the want of any advantage on the crossed side is far from certain: with Thunbergia the parent-plants were in an odd semi-sterile condition, and the offspring grew very unequally; with Hibiscus and Apium much too few plants were raised for the measurements to be trusted, and the cross-fertilised flowers of Hibiscus produced rather more seed than did the self-fertilised; with Vandellia the crossed plants were a little taller and heavier than the self-fertilised, but as they were less fertile the case must be left doubtful. Lastly, with Pisum, Primula, the three generations of Canna, and the three of Nicotiana (which together complete the twelve cases), a cross between two plants certainly did no good or very little good to the offspring; but we have reason to believe that this is the result of these plants having been self-fertilised and cultivated under nearly uniform conditions for several generations. The same result followed with the experimental plants of Ipomoea and Mimulus, and to a certain extent with some other species, which had been intentionally treated by me in this manner; yet we know that these species in their normal condition profit greatly by being intercrossed. There is, therefore, not a single case in Table 7/A which affords decisive evidence against the rule that a cross between plants, the progenitors of which have been subjected to somewhat diversified conditions, is beneficial to the offspring. This is a surprising conclusion, for from the a.n.a.logy of domesticated animals it could not have been antic.i.p.ated, that the good effects of crossing or the evil effects of self-fertilisation would have been perceptible until the plants had been thus treated for several generations.

The results given in Table 7/A may be looked at under another point of view. Hitherto each generation has been considered as a separate case, of which there are eighty-three; and this no doubt is the more correct method of comparing the crossed and self-fertilised plants.

But in those cases in which plants of the same species were observed during several generations, a general average of their heights in all the generations together may be made; and such averages are given in Table 7/A; for instance, under Ipomoea the general average for the plants of all ten generations is as 100 for the crossed, to 77 for the self-fertilised plants. This having been done in each case in which more than one generation was raised, it is easy to calculate the average of the average heights of the crossed and self-fertilised plants of all the species included in Table 7/A. It should however be observed that as only a few plants of some species, whilst a considerable number of others, were measured, the value of the mean or average heights of the several species is very different. Subject to this source of error, it may be worth while to give the mean of the mean heights of the fifty-four species in Table 7/A; and the result is, calling the mean of the mean heights of the crossed plants 100, that of the self-fertilised plants is 87. But it is a better plan to divide the fifty-four species into three groups, as was done with the previously given eighty-three cases. The first group consists of species of which the mean heights of the self-fertilised plants are within five per cent of 100; so that the crossed and self-fertilised plants are approximately equal; and of such species there are twelve about which nothing need be said, the mean of the mean heights of the self-fertilised being of course very nearly 100, or exactly 99.58. The second group consists of the species, thirty-seven in number, of which the mean heights of the crossed plants exceed that of the self-fertilised plants by more than five per cent; and the mean of their mean heights is to that of the self-fertilised plants as 100 to 78. The third group consists of the species, only five in number, of which the mean heights of the self-fertilised plants exceed that of the crossed by more than five per cent; and here the mean of the mean heights of the crossed plants is to that of the self-fertilised as 100 to 109. Therefore if we exclude the species which are approximately equal, there are thirty-seven species in which the mean of the mean heights of the crossed plants exceeds that of the self-fertilised by twenty-two per cent; whereas there are only five species in which the mean of the mean heights of the self-fertilised plants exceeds that of the crossed, and this only by nine per cent.

The truth of the conclusion--that the good effects of a cross depend on the plants having been subjected to different conditions or to their belonging to different varieties, in both of which cases they would almost certainly differ somewhat in const.i.tution--is supported by a comparison of the Tables 7/A and 7/C. The latter table gives the results of crossing plants with a fresh stock or with a distinct variety; and the superiority of the crossed offspring over the self-fertilised is here much more general and much more strongly marked than in Table 7/A, in which plants of the same stock were crossed. We have just seen that the mean of the mean heights of the crossed plants of the whole fifty-four species in Table 7/A is to that of the self-fertilised plants as 100 to 87; whereas the mean of the mean heights of the plants crossed by a fresh stock is to that of the self-fertilised in Table 7/C as 100 to 74. So that the crossed plants beat the self-fertilised plants by thirteen per cent in Table 7/A, and by twenty-six per cent, or double as much, in Table 7/C, which includes the results of the cross by a fresh stock.

TABLE 7/B.

A few words must be added on the weights of the crossed plants of the same stock, in comparison with the self-fertilised. Eleven cases are given in Table 7/B, relating to eight species. The number of plants which were weighed is shown in the two left columns, and their relative weights in the right column, that of the crossed plants being taken as 100. A few other cases have already been recorded in Table 7/C in reference to plants crossed by a fresh stock. I regret that more trials of this kind were not made, as the evidence of the superiority of the crossed over the self-fertilised plants is thus shown in a more conclusive manner than by their relative heights. But this plan was not thought of until a rather late period, and there were difficulties either way, as the seeds had to be collected when ripe, by which time the plants had often begun to wither. In only one out of the eleven cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants exceed the crossed in weight; and we have already seen they are likewise superior to them in height, though inferior in fertility, the whole advantage of a cross being here confined to the reproductive system.

With Vandellia the crossed plants were a little heavier, as they were also a little taller than the self-fertilised; but as a greater number of more productive capsules were produced by the cleistogene flowers on the self-fertilised plants than by those on the crossed plants, the case must be left, as remarked under Table 7/A, altogether doubtful. The crossed and self-fertilised offspring from a partially self-sterile plant of Reseda odorata were almost equal in weight, though not in height. In the remaining eight cases, the crossed plants show a wonderful superiority over the self-fertilised, being more than double their weight, except in one case, and here the ratio is as high as 100 to 67. The results thus deduced from the weights of the plants confirm in a striking manner the former evidence of the beneficial effects of a cross between two plants of the same stock; and in the few cases in which plants derived from a cross with a fresh stock were weighed, the results are similar or even more striking.

CHAPTER VIII.

DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSt.i.tUTIONAL VIGOUR AND IN OTHER RESPECTS.

Greater const.i.tutional vigour of crossed plants.

The effects of great crowding.

Compet.i.tion with other kinds of plants.

Self-fertilised plants more liable to premature death.

Crossed plants generally flower before the self-fertilised.

Negative effects of intercrossing flowers on the same plant.

Cases described.

Transmission of the good effects of a cross to later generations.

Effects of crossing plants of closely related parentage.

Uniform colour of the flowers on plants self-fertilised during several generations and cultivated under similar conditions.

GREATER CONSt.i.tUTIONAL VIGOUR OF CROSSED PLANTS.

As in almost all my experiments an equal number of crossed and self-fertilised seeds, or more commonly seedlings just beginning to sprout, were planted on the opposite sides of the same pots, they had to compete with one another; and the greater height, weight, and fertility of the crossed plants may be attributed to their possessing greater innate const.i.tutional vigour. Generally the plants of the two lots whilst very young were of equal height; but afterwards the crossed gained insensibly on their opponents, and this shows that they possessed some inherent superiority, though not displayed at a very early period in life. There were, however, some conspicuous exceptions to the rule of the two lots being at first equal in height; thus the crossed seedlings of the broom (Sarothamnus scoparius) when under three inches in height were more than twice as tall as the self-fertilised plants.

After the crossed or the self-fertilised plants had once grown decidedly taller than their opponents, a still increasing advantage would tend to follow from the stronger plants robbing the weaker ones of nourishment and overshadowing them. This was evidently the case with the crossed plants of Viola tricolor, which ultimately quite overwhelmed the self-fertilised. But that the crossed plants have an inherent superiority, independently of compet.i.tion, was sometimes well shown when both lots were planted separately, not far distant from one another, in good soil in the open ground. This was likewise shown in several cases, even with plants growing in close compet.i.tion with one another, by one of the self-fertilised plants exceeding for a time its crossed opponent, which had been injured by some accident or was at first sickly, but being ultimately conquered by it. The plants of the eighth generation of Ipomoea were raised from small seeds produced by unhealthy parents, and the self-fertilised plants grew at first very rapidly, so that when the plants of both lots were about three feet in height, the mean height of the crossed to that of the self-fertilised was as 100 to 122; when they were about six feet high the two lots were very nearly equal, but ultimately when between eight and nine feet in height, the crossed plants a.s.serted their usually superiority, and were to the self-fertilised in height as 100 to 85.

The const.i.tutional superiority of the crossed over the self-fertilised plants was proved in another way in the third generation of Mimulus, by self-fertilised seeds being sown on one side of a pot, and after a certain interval of time crossed seeds on the opposite side. The self-fertilised seedlings thus had (for I ascertained that the seeds germinated simultaneously) a clear advantage over the crossed in the start for the race. Nevertheless they were easily beaten (as may be seen under the head of Mimulus) when the crossed seeds were sown two whole days after the self-fertilised. But when the interval was four days, the two lots were nearly equal throughout life. Even in this latter case the crossed plants still possessed an inherent advantage, for after both lots had grown to their full height they were cut down, and without being disturbed were transferred to a larger pot, and when in the ensuing year they had again grown to their full height they were measured; and now the tallest crossed plants were to the tallest self-fertilised plants in height as 100 to 75, and in fertility (i.e., by weight of seeds produced by an equal number of capsules from both lots) as 100 to 34.

My usual method of proceeding, namely, to plant several pairs of crossed and self-fertilised seeds in an equal state of germination on the opposite sides of the same pots, so that the plants were subjected to moderately severe mutual compet.i.tion, was I think the best that could have been followed, and was a fair test of what occurs in a state of nature. For plants sown by nature generally come up crowded, and are almost always exposed to very severe compet.i.tion with one another and with other kinds of plants. This latter consideration led me to make some trials, chiefly but not exclusively with Ipomoea and Mimulus, by sowing crossed and self-fertilised seeds on the opposite sides of large pots in which other plants had long been growing, or in the midst of other plants out of doors. The seedlings were thus subjected to very severe compet.i.tion with plants of other kinds; and in all such cases, the crossed seedlings exhibited a great superiority in their power of growth over the self-fertilised.

After the germinating seedlings had been planted in pairs on the opposite sides of several pots, the remaining seeds, whether or not in a state of germination, were in most cases sown very thickly on the two sides of an additional large pot; so that the seedlings came up extremely crowded, and were subjected to extremely severe compet.i.tion and unfavourable conditions. In such cases the crossed plants almost invariably showed a greater superiority over the self-fertilised, than did the plants which grew in pairs in the pots.