The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom · Charles Darwin
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation injurious. Allied species differ greatly in the means by which cross-fertilisation is favoured and self-fertilisation avoided. The benefits and evils of the two processes depend on the degree of differentiation in the sexual elements. The evil effects not due to the combination of morbid tendencies in the parents. Nature of the conditions to which plants are subjected when growing near together in a state of nature or under culture, and the effects of such conditions. Theoretical considerations with respect to the interaction of differentiated sexual elements. Practical lessons. Genesis of the two sexes. Close correspondence between the effects of cross-fertilisation and self-fertilisation, and of the legitimate and illegitimate unions of heterostyled plants, in comparison with hybrid unions.
The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations. So it must be, owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together. The same inference may be drawn still more surely with respect to those plants, such as Reseda and Eschscholtzia, which are sterile with their own pollen, but fertile with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in my experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation.
With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomoea were intercrossed for nine successive generations; they were then again intercrossed, and at the same time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An analogous experiment with Eschscholtzia gave a similar result, as far as fertility was concerned. In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was injurious; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly parallel case. With various other plants, the wonderful effects of a cross with a fresh stock may be seen in Table 7/C. Several accounts have also been published of the extraordinary growth of seedlings from a cross between two varieties of the same species, some of which are known never to fertilise themselves; so that here neither self-fertilisation nor relationship even in a remote degree can have come into play. (12/1. See ‘Variation under Domestication’ chapter 19 2nd edition volume 2 page 159.) We may therefore conclude that the above two propositions are true,--that cross-fertilisation is generally beneficial and self-fertilisation injurious to the offspring.
That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals; but then we must remember that the closest possible interbreeding with such animals, that is, between brothers and sisters, cannot be considered as nearly so close a union as that between the pollen and ovules of the same flower. Whether the evil from self-fertilisation goes on increasing during successive generations is not as yet known; but we may infer from my experiments that the increase if any is far from rapid. After plants have been propagated by self-fertilisation for several generations, a single cross with a fresh stock restores their pristine vigour; and we have a strictly analogous result with our domestic animals. (12/2. Ibid chapter 19 2nd edition volume 2 page 159.) The good effects of cross-fertilisation are transmitted by plants to the next generation; and judging from the varieties of the common pea, to many succeeding generations. But this may merely be that crossed plants of the first generation are extremely vigorous, and transmit their vigour, like any other character, to their successors.
Notwithstanding the evil which many plants suffer from self-fertilisation, they can be thus propagated under favourable conditions for many generations, as shown by some of my experiments, and more especially by the survival during at least half a century of the same varieties of the common pea and sweet-pea. The same conclusion probably holds good with several other exotic plants, which are never or most rarely cross-fertilised in this country. But all these plants, as far as they have been tried, profit greatly by a cross with a fresh stock. Some few plants, for instance, Ophrys apifera, have almost certainly been propagated in a state of nature for thousands of generations without having been once intercrossed; and whether they would profit by a cross with a fresh stock is not known. But such cases ought not to make us doubt that as a general rule crossing is beneficial, any more than the existence of plants which, in a state of nature, are propagated exclusively by rhizomes, stolons, etc. (their flowers never producing seeds), (12/3. I have given several cases in my ‘Variation under Domestication’ chapter 18 2nd edition volume 2 page 152.) (their flowers never producing seeds), should make us doubt that seminal generation must have some great advantage, as it is the common plan followed by nature. Whether any species has been reproduced asexually from a very remote period cannot, of course, be ascertained. Our sole means for forming any judgment on this head is the duration of the varieties of our fruit trees which have been long propagated by grafts or buds. Andrew Knight formerly maintained that under these circumstances they always become weakly, but this conclusion has been warmly disputed by others. A recent and competent judge, Professor Asa Gray, leans to the side of Andrew Knight, which seems to me, from such evidence as I have been able to collect, the more probable view, notwithstanding many opposed facts. (12/4. ‘Darwiniana: Essays and Reviews pertaining to Darwinism’ 1876 page 338.)
The means for favouring cross-fertilisation and preventing self-fertilisation, or conversely for favouring self-fertilisation and preventing to a certain extent cross-fertilisation, are wonderfully diversified; and it is remarkable that these differ widely in closely allied plants,--in the species of the same genus, and sometimes in the individuals of the same species. (12/5. Hildebrand has insisted strongly to this effect in his valuable observations on the fertilisation of the Gramineae: ‘Monatsbericht K. Akad. Berlin’ October 1872 page 763.) It is not rare to find hermaphrodite plants and others with separated sexes within the same genus; and it is common to find some of the species dichogamous and others maturing their sexual elements simultaneously. The dichogamous genus Saxifraga contains proterandrous and proterogynous species. (12/6. Dr. Engler ‘Botanische Zeitung’ 1868 page 833.) Several genera include both heterostyled (dimorphic or trimorphic forms) and homostyled species. Ophrys offers a remarkable instance of one species having its structure manifestly adapted for self-fertilisation, and other species as manifestly adapted for cross-fertilisation. Some con-generic species are quite sterile and others quite fertile with their own pollen. From these several causes we often find within the same genus species which do not produce seeds, while others produce an abundance, when insects are excluded. Some species bear cleistogene flowers which cannot be crossed, as well as perfect flowers, whilst others in the same genus never produce cleistogene flowers. Some species exist under two forms, the one bearing conspicuous flowers adapted for cross-fertilisation, the other bearing inconspicuous flowers adapted for self-fertilisation, whilst other species in the same genus present only a single form. Even with the individuals of the same species, the degree of self-sterility varies greatly, as in Reseda. With polygamous plants, the distribution of the sexes differs in the individuals of the same species. The relative period at which the sexual elements in the same flower are mature, differs in the varieties of Pelargonium; and Carriere gives several cases, showing that the period varies according to the temperature to which the plants are exposed. (12/7. ‘Des Varieties’ 1865 page 30.)
This extraordinary diversity in the means for favouring or preventing cross- and self-fertilisation in closely allied forms, probably depends on the results of both processes being highly beneficial to the species, but directly opposed in many ways to one another and dependent on variable conditions. Self-fertilisation assures the production of a large supply of seeds; and the necessity or advantage of this will be determined by the average length of life of the plant, which largely depends on the amount of destruction suffered by the seeds and seedlings. This destruction follows from the most various and variable causes, such as the presence of animals of several kinds, and the growth of surrounding plants. The possibility of cross-fertilisation depends mainly on the presence and number of certain insects, often of insects belonging to special groups, and on the degree to which they are attracted to the flowers of any particular species in preference to other flowers,--all circumstances likely to change. Moreover, the advantages which follow from cross-fertilisation differ much in different plants, so that it is probable that allied plants would often profit in different degrees by cross-fertilisation. Under these extremely complex and fluctuating conditions, with two somewhat opposed ends to be gained, namely, the safe propagation of the species and the production of cross-fertilised, vigorous offspring, it is not surprising that allied forms should exhibit an extreme diversity in the means which favour either end. If, as there is reason to suspect, self-fertilisation is in some respects beneficial, although more than counterbalanced by the advantages derived from a cross with a fresh stock, the problem becomes still more complicated.
As I only twice experimented on more than a single species in a genus, I cannot say whether the crossed offspring of the several species within the same genus differ in their degree of superiority over their self-fertilised brethren; but I should expect that this would often prove to be the case from what was observed with the two species of Lobelia and with the individuals of the same species of Nicotiana. The species belonging to distinct genera in the same family certainly differ in this respect. The effects of cross- and self-fertilisation may be confined either to the growth or to the fertility of the offspring, but generally extends to both qualities. There does not seem to exist any close correspondence between the degree to which their offspring profit by this process; but we may easily err on this head, as there are two means for ensuring cross-fertilisation which are not externally perceptible, namely, self-sterility and the prepotent fertilising influence of pollen from another individual. Lastly, it has been shown in a former chapter that the effect produced by cross and self-fertilisation on the fertility of the parent-plants does not always correspond with that produced on the height, vigour, and fertility of their offspring. The same remark applies to crossed and self-fertilised seedlings when these are used as the parent-plants. This want of correspondence probably depends, at least in part, on the number of seeds produced being chiefly determined by the number of the pollen-tubes which reach the ovules, and this will be governed by the reaction between the pollen and the stigmatic secretion or tissues; whereas the growth and constitutional vigour of the offspring will be chiefly determined, not only by the number of pollen-tubes reaching the ovules, but by the nature of the reaction between the contents of the pollen-grains and ovules.
There are two other important conclusions which may be deduced from my observations: firstly, that the advantages of cross-fertilisation do not follow from some mysterious virtue in the mere union of two distinct individuals, but from such individuals having been subjected during previous generations to different conditions, or to their having varied in a manner commonly called spontaneous, so that in either case their sexual elements have been in some degree differentiated. And secondly, that the injury from self-fertilisation follows from the want of such differentiation in the sexual elements. These two propositions are fully established by my experiments. Thus, when plants of the Ipomoea and of the Mimulus, which had been self-fertilised for the seven previous generations and had been kept all the time under the same conditions, were intercrossed one with another, the offspring did not profit in the least by the cross.
Mimulus offers another instructive case, showing that the benefit of a cross depends on the previous treatment of the progenitors: plants which had been self-fertilised for the eight previous generations were crossed with plants which had been intercrossed for the same number of generations, all having been kept under the same conditions as far as possible; seedlings from this cross were grown in competition with others derived from the same self-fertilised mother-plant crossed by a fresh stock; and the latter seedlings were to the former in height as 100 to 52, and in fertility as 100 to 4. An exactly parallel experiment was tried on Dianthus, with this difference, that the plants had been self-fertilised only for the three previous generations, and the result was similar though not so strongly marked. The foregoing two cases of the offspring of Ipomoea and Eschscholtzia, derived from a cross with a fresh stock, being as much superior to the intercrossed plants of the old stock, as these latter were to the self-fertilised offspring, strongly supports the same conclusion. A cross with a fresh stock or with another variety seems to be always highly beneficial, whether or not the mother-plants have been intercrossed or self-fertilised for several previous generations. The fact that a cross between two flowers on the same plant does no good or very little good, is likewise a strong corroboration of our conclusion; for the sexual elements in the flowers on the same plant can rarely have been differentiated, though this is possible, as flower-buds are in one sense distinct individuals, sometimes varying and differing from one another in structure or constitution. Thus the proposition that the benefit from cross-fertilisation depends on the plants which are crossed having been subjected during previous generations to somewhat different conditions, or to their having varied from some unknown cause as if they had been thus subjected, is securely fortified on all sides.
Before proceeding any further, the view which has been maintained by several physiologists must be noticed, namely, that all the evils from breeding animals too closely, and no doubt, as they would say, from the self-fertilisation of plants, is the result of the increase of some morbid tendency or weakness of constitution common to the closely related parents, or to the two sexes of hermaphrodite plants. Undoubtedly injury has often thus resulted; but it is a vain attempt to extend this view to the numerous cases given in my Tables. It should be remembered that the same mother-plant was both self-fertilised and crossed, so that if she had been unhealthy she would have transmitted half her morbid tendencies to her crossed offspring. But plants appearing perfectly healthy, some of them growing wild, or the immediate offspring of wild plants, or vigorous common garden-plants, were selected for experiment. Considering the number of species which were tried, it is nothing less than absurd to suppose that in all these cases the mother-plants, though not appearing in any way diseased, were weak or unhealthy in so peculiar a manner that their self-fertilised seedlings, many hundreds in number, were rendered inferior in height, weight, constitutional vigour and fertility to their crossed offspring. Moreover, this belief cannot be extended to the strongly marked advantages which invariably follow, as far as my experience serves, from intercrossing the individuals of the same variety or of distinct varieties, if these have been subjected during some generations to different conditions.
It is obvious that the exposure of two sets of plants during several generations to different conditions can lead to no beneficial results, as far as crossing is concerned, unless their sexual elements are thus affected. That every organism is acted on to a certain extent by a change in its environment, will not, I presume, be disputed. It is hardly necessary to advance evidence on this head; we can perceive the difference between individual plants of the same species which have grown in somewhat more shady or sunny, dry or damp places. Plants which have been propagated for some generations under different climates or at different seasons of the year transmit different constitutions to their seedlings. Under such circumstances, the chemical constitution of their fluids and the nature of their tissues are often modified. (12/8. Numerous cases together with references are given in my ‘Variation under Domestication’ chapter 23 2nd edition volume 2 page 264. With respect to animals, Mr. Brackenridge ‘A Contribution to the Theory of Diathesis’ Edinburgh 1869, has well shown that the different organs of animals are excited into different degrees of activity by differences of temperature and food, and become to a certain extent adapted to them.) Many other such facts could be adduced. In short, every alteration in the function of a part is probably connected with some corresponding, though often quite imperceptible change in structure or composition.
Whatever affects an organism in any way, likewise tends to act on its sexual elements. We see this in the inheritance of newly acquired modifications, such as those from the increased use or disuse of a part, and even from mutilations if followed by disease. (12/9. ‘Variation under Domestication’ chapter 12 2nd edition volume 1 page 466.) We have abundant evidence how susceptible the reproductive system is to changed conditions, in the many instances of animals rendered sterile by confinement; so that they will not unite, or if they unite do not produce offspring, though the confinement may be far from close; and of plants rendered sterile by cultivation. But hardly any cases afford more striking evidence how powerfully a change in the conditions of life acts on the sexual elements, than those already given, of plants which are completely self-sterile in one country, and when brought to another, yield, even in the first generation, a fair supply of self-fertilised seeds. But it may be said, granting that changed conditions act on the sexual elements, how can two or more plants growing close together, either in their native country or in a garden, be differently acted on, inasmuch as they appear to be exposed to exactly the same conditions? Although this question has been already considered, it deserves further consideration under several points of view. In my experiments with Digitalis purpurea, some flowers on a wild plant were self-fertilised, and others were crossed with pollen from another plant growing within two or three feet’s distance. The crossed and self-fertilised plants raised from the seeds thus obtained, produced flower-stems in number as 100 to 47, and in average height as 100 to 70. Therefore the cross between these two plants was highly beneficial; but how could their sexual elements have been differentiated by exposure to different conditions?
If the progenitors of the two plants had lived on the same spot during the last score of generations, and had never been crossed with any plant beyond the distance of a few feet, in all probability their offspring would have been reduced to the same state as some of the plants in my experiments,--such as the intercrossed plants of the ninth generation of Ipomoea,--or the self-fertilised plants of the eighth generation of Mimulus,--or the offspring from flowers on the same plant,--and in this case a cross between the two plants of Digitalis would have done no good. But seeds are often widely dispersed by natural means, and one of the above two plants or one of their ancestors may have come from a distance, from a more shady or sunny, dry or moist place, or from a different kind of soil containing other organic or inorganic matter. We know from the admirable researches of Messrs. Lawes and Gilbert that different plants require and consume very different amounts of inorganic matter.
(12/10. ‘Journal of the Royal Agricultural Society of England’ volume 24 part 1.) But the amount in the soil would probably not make so great a difference to the several individuals of any particular species as might at first be expected; for the surrounding species with different requirements would tend, from existing in greater or lesser numbers, to keep each species in a sort of equilibrium, with respect to what it could obtain from the soil. So it would be even with respect to moisture during dry seasons; and how powerful is the influence of a little more or less moisture in the soil on the presence and distribution of plants, is often well shown in old pasture fields which still retain traces of former ridges and furrows. Nevertheless, as the proportional numbers of the surrounding plants in two neighbouring places is rarely exactly the same, the individuals of the same species will be subjected to somewhat different conditions with respect to what they can absorb from the soil. It is surprising how the free growth of one set of plants affects others growing mingled with them; I allowed the plants on rather more than a square yard of turf which had been closely mown for several years, to grow up; and nine species out of twenty were thus exterminated; but whether this was altogether due to the kinds which grew up robbing the others of nutriment, I do not know.
Seeds often lie dormant for several years in the ground, and germinate when brought near the surface by any means, as by burrowing animals. They would probably be affected by the mere circumstance of having long lain dormant; for gardeners believe that the production of double flowers and of fruit is thus influenced. Seeds, moreover, which were matured during different seasons, will have been subjected during the whole course of their development to different degrees of heat and moisture. It was shown in the last chapter that pollen is often carried by insects to a considerable distance from plant to plant. Therefore one of the parents or ancestors of our two plants of Digitalis may have been crossed by a distant plant growing under somewhat different conditions. Plants thus crossed often produce an unusually large number of seeds; a striking instance of this fact is afforded by the Bignonia, previously mentioned, which was fertilised by Fritz Muller with pollen from some adjoining plants and set hardly any seed, but when fertilised with pollen from a distant plant, was highly fertile. Seedlings from a cross of this kind grow with great vigour, and transmit their vigour to their descendants. These, therefore, in the struggle for life, will generally beat and exterminate the seedlings from plants which have long grown near together under the same conditions, and will thus tend to spread.
Text from Project Gutenberg, public domain in the United States.