The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗
- Author: Charles Robert Darwin
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The illegitimate plants derived from both forms flowered later than the legitimate, and were to the latter in height as 69 to 100. But as these illegitimate plants were descended from parents fertilised with their own pollen, whilst the legitimate plants were descended from parents crossed with pollen from a distinct individual, it is impossible to know how much of their difference in height and period of flowering, is due to the illegitimate birth of the one set, and how much to the other set being the product of a cross between distinct plants.]
CONCLUDING REMARKS ON THE ILLEGITIMATE OFFSPRING OF HETEROSTYLED TRIMORPHIC AND DIMORPHIC PLANTS.
It is remarkable how closely and in how many points illegitimate unions between the two or three forms of the same heterostyled species, together with their illegitimate offspring, resemble hybrid unions between distinct species together with their hybrid offspring. In both cases we meet with every degree of sterility, from very slightly lessened fertility to absolute barrenness, when not even a single seed-capsule is produced. In both cases the facility of effecting the first union is much influenced by the conditions to which the plants are exposed. (5/14. This has been remarked by many experimentalists in effecting crosses between distinct species; and in regard to illegitimate unions I have given in the first chapter a striking illustration in the case of Primula veris.) Both with hybrids and illegitimate plants the innate degree of sterility is highly variable in plants raised from the same mother-plant. In both cases the male organs are more plainly affected than the female; and we often find contabescent anthers enclosing shrivelled and utterly powerless pollen-grains. The more sterile hybrids, as Max Wichura has well shown, are sometimes much dwarfed in stature, and have so weak a constitution that they are liable to premature death (5/15. 'Die Bastardbefruchtung im Pflanzenreich' 1865.); and we have seen exactly parallel cases with the illegitimate seedlings of Lythrum and Primula. Many hybrids are the most persistent and profuse flowerers, as are some illegitimate plants. When a hybrid is crossed by either pure parent-form, it is notoriously much more fertile than when crossed inter se or by another hybrid; so when an illegitimate plant is fertilised by a legitimate plant, it is more fertile than when fertilised inter se or by another illegitimate plant. When two species are crossed and they produce numerous seeds, we expect as a general rule that their hybrid offspring will be moderately fertile; but if the parent species produce extremely few seeds, we expect that the hybrids will be very sterile. But there are marked exceptions, as shown by Gartner, to these rules. So it is with illegitimate unions and illegitimate offspring. Thus the mid- styled form of Lythrum salicaria, when illegitimately fertilised with pollen from the longest stamens of the short-styled form, produced an unusual number of seeds; and their illegitimate offspring were not at all, or hardly at all, sterile. On the other hand, the illegitimate offspring from the long-styled form, fertilised with pollen from the shortest stamens of the same form, yielded few seeds, and the illegitimate offspring thus produced were very sterile; but they were more sterile than might have been expected relatively to the difficulty of effecting the union of the parent sexual elements. No point is more remarkable in regard to the crossing of species than their unequal reciprocity. Thus species A will fertilise B with the greatest ease; but B will not fertilise A after hundreds of trials. We have exactly the same case with illegitimate unions; for the mid-styled Lythrum salicaria was easily fertilised by pollen from the longest stamens of the short-styled form, and yielded many seeds; but the latter form did not yield a single seed when fertilised by the longest stamens of the mid-styled form.
Another important point is prepotency. Gartner has shown that when a species is fertilised with pollen from another species, if it be afterwards fertilised with its own pollen, or with that of the same species, this is so prepotent over the foreign pollen that the effect of the latter, though placed on the stigma some time previously, is entirely destroyed. Exactly the same thing occurs with the two forms of a heterostyled species. Thus several long-styled flowers of Primula veris were fertilised illegitimately with pollen from another plant of the same form, and twenty-four hours afterwards legitimately with pollen from a short- styled dark-red polyanthus which is a variety of P. veris; and the result was that every one of the thirty seedlings thus raised bore flowers more or less red, showing plainly how prepotent the legitimate pollen from a short-styled plant was over the illegitimate pollen from a long-styled plant.
In all the several foregoing points the parallelism is wonderfully close between the effects of illegitimate and hybrid fertilisation. It is hardly an exaggeration to assert that seedlings from an illegitimately fertilised heterostyled plant are hybrids formed within the limits of one and the same species. This conclusion is important, for we thus learn that the difficulty in sexually uniting two organic forms and the sterility of their offspring, afford no sure criterion of so-called specific distinctness. If any one were to cross two varieties of the same form of Lythrum or Primula for the sake of ascertaining whether they were specifically distinct, and he found that they could be united only with some difficulty, that their offspring were extremely sterile, and that the parents and their offspring resembled in a whole series of relations crossed species and their hybrid offspring, he might maintain that his varieties had been proved to be good and true species; but he would be completely deceived. In the second place, as the forms of the same trimorphic or dimorphic heterostyled species are obviously identical in general structure, with the exception of the reproductive organs, and as they are identical in general constitution (for they live under precisely the same conditions), the sterility of their illegitimate unions and that of their illegitimate offspring, must depend exclusively on the nature of the sexual elements and on their incompatibility for uniting in a particular manner. And as we have just seen that distinct species when crossed resemble in a whole series of relations the forms of the same species when illegitimately united, we are led to conclude that the sterility of the former must likewise depend exclusively on the incompatible nature of their sexual elements, and not on any general difference in constitution or structure. We are, indeed, led to this same conclusion by the impossibility of detecting any differences sufficient to account for certain species crossing with the greatest ease, whilst other closely allied species cannot be crossed, or can be crossed only with extreme difficulty. We are led to this conclusion still more forcibly by considering the great difference which often exists in the facility of crossing reciprocally the same two species; for it is manifest in this case that the result must depend on the nature of the sexual elements, the male element of the one species acting freely on the female element of the other, but not so in a reversed direction. And now we see that this same conclusion is independently and strongly fortified by the consideration of the illegitimate unions of trimorphic and dimorphic heterostyled plants. In so complex and obscure a subject as hybridism it is no slight gain to arrive at a definite conclusion, namely, that we must look exclusively to functional differences in the sexual elements, as the cause of the sterility of species when first crossed and of their hybrid offspring. It was this consideration which led me to make the many observations recorded in this chapter, and which in my opinion make them worthy of publication.
CHAPTER VI. CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants. Summary of the differences in fertility between legitimately and illegitimately fertilised plants. Diameter of the pollen-grains, size of anthers and structure of stigma in the different forms. Affinities of the genera which include heterostyled species. Nature of the advantages derived from heterostylism. The means by which plants became heterostyled. Transmission of form. Equal-styled varieties of heterostyled plants. Final remarks.
In the foregoing chapters all the heterostyled plants known to me have been more or less fully described. Several other cases have been indicated, especially by Professor Asa Gray and Kuhn, in which the individuals of the same species differ in the length of their stamens and pistils (6/1. Asa Gray 'American Journal of Science' 1865 page 101 and elsewhere as already referred to. Kuhn 'Botanische Zeitung' 1867 page 67.); but as I have been often deceived by this character taken alone, it seems to me the more prudent course not to rank any species as heterostyled, unless we have evidence of more important differences between the forms, as in the diameter of the pollen-grains, or in the structure of the stigma. The individuals of many ordinary hermaphrodite plants habitually fertilise one another, owing to their male and female organs being mature at different periods, or to the structure of the parts, or to self-sterility, etc.; and so it is with many hermaphrodite animals, for instance, land-snails or earth-worms; but in all these cases any one individual can fully fertilise or be fertilised by any other individual of the same species. This is not so with heterostyled plants: a long-styled, mid-styled or short-styled plant cannot fully fertilise or be fertilised by any other individual, but only by one belonging to another form. Thus the essential character of plants belonging to the heterostyled class is that the individuals are divided into two or three bodies, like the males and females of dioecious plants or of the higher animals, which exist in approximately equal numbers and are adapted for reciprocal fertilisation. The existence, therefore, of two or three bodies of individuals, differing from one another in the above more important characteristics, offers by itself good evidence that the species is heterostyled. But absolutely conclusive evidence can be derived only from experiments, and by finding that pollen must be applied from the one form to the other in order to ensure complete fertility.
In order to show how much more fertile each form is when legitimately fertilised with pollen from the other form (or in the case of trimorphic species, with the proper pollen from one of the two other forms) than when illegitimately fertilised with its own-form pollen, I will append Table 6.33 giving a summary of the results in all the cases hitherto ascertained. The fertility of the unions may be judged by two standards, namely, by the proportion of flowers which, when fertilised in the two methods, yield capsules, and by the average number of seeds per capsule. When there is a dash in the left hand column opposite to the name of the species, the proportion of the flowers which yielded capsules was not recorded.
TABLE 6.33. Fertility of the legitimate unions taken together, compared with that of the illegitimate unions together. The fertility of the legitimate unions, as judged by both standards, is taken as 100.
Column 1: Name of species. Column 2: Illegitimate unions : proportional number of flowers which produced capsules. Column 3: Illegitimate unions : average number of seeds per capsule.
Primula veris : 69 : 65.
Primula elatior : 27 : 75.
Primula vulgaris : 60 : 54.
Primula Sinensis : 84 : 63.
Primula Sinensis (second trial) : 0 : 53.
Primula Sinensis (Hildebrand) : 100 : 42.
Primula auricula (Scott) : 80 : 15.
Primula Sikkimensis (Scott) : 95 : 31.
Primula cortusoides (Scott) : 74 : 66.
Primula involucrata (Scott) : 72 : 48.
Primula farinosa (Scott) : 71 : 44.
Average of the nine species of Primula : 88.4 : 69.
Hottonia palustris (H. Muller) : - : 61.
Linum grandiflorum (the difference probably is much greater) : - : 69.
Linum perenne : - : 20.
Linum
CONCLUDING REMARKS ON THE ILLEGITIMATE OFFSPRING OF HETEROSTYLED TRIMORPHIC AND DIMORPHIC PLANTS.
It is remarkable how closely and in how many points illegitimate unions between the two or three forms of the same heterostyled species, together with their illegitimate offspring, resemble hybrid unions between distinct species together with their hybrid offspring. In both cases we meet with every degree of sterility, from very slightly lessened fertility to absolute barrenness, when not even a single seed-capsule is produced. In both cases the facility of effecting the first union is much influenced by the conditions to which the plants are exposed. (5/14. This has been remarked by many experimentalists in effecting crosses between distinct species; and in regard to illegitimate unions I have given in the first chapter a striking illustration in the case of Primula veris.) Both with hybrids and illegitimate plants the innate degree of sterility is highly variable in plants raised from the same mother-plant. In both cases the male organs are more plainly affected than the female; and we often find contabescent anthers enclosing shrivelled and utterly powerless pollen-grains. The more sterile hybrids, as Max Wichura has well shown, are sometimes much dwarfed in stature, and have so weak a constitution that they are liable to premature death (5/15. 'Die Bastardbefruchtung im Pflanzenreich' 1865.); and we have seen exactly parallel cases with the illegitimate seedlings of Lythrum and Primula. Many hybrids are the most persistent and profuse flowerers, as are some illegitimate plants. When a hybrid is crossed by either pure parent-form, it is notoriously much more fertile than when crossed inter se or by another hybrid; so when an illegitimate plant is fertilised by a legitimate plant, it is more fertile than when fertilised inter se or by another illegitimate plant. When two species are crossed and they produce numerous seeds, we expect as a general rule that their hybrid offspring will be moderately fertile; but if the parent species produce extremely few seeds, we expect that the hybrids will be very sterile. But there are marked exceptions, as shown by Gartner, to these rules. So it is with illegitimate unions and illegitimate offspring. Thus the mid- styled form of Lythrum salicaria, when illegitimately fertilised with pollen from the longest stamens of the short-styled form, produced an unusual number of seeds; and their illegitimate offspring were not at all, or hardly at all, sterile. On the other hand, the illegitimate offspring from the long-styled form, fertilised with pollen from the shortest stamens of the same form, yielded few seeds, and the illegitimate offspring thus produced were very sterile; but they were more sterile than might have been expected relatively to the difficulty of effecting the union of the parent sexual elements. No point is more remarkable in regard to the crossing of species than their unequal reciprocity. Thus species A will fertilise B with the greatest ease; but B will not fertilise A after hundreds of trials. We have exactly the same case with illegitimate unions; for the mid-styled Lythrum salicaria was easily fertilised by pollen from the longest stamens of the short-styled form, and yielded many seeds; but the latter form did not yield a single seed when fertilised by the longest stamens of the mid-styled form.
Another important point is prepotency. Gartner has shown that when a species is fertilised with pollen from another species, if it be afterwards fertilised with its own pollen, or with that of the same species, this is so prepotent over the foreign pollen that the effect of the latter, though placed on the stigma some time previously, is entirely destroyed. Exactly the same thing occurs with the two forms of a heterostyled species. Thus several long-styled flowers of Primula veris were fertilised illegitimately with pollen from another plant of the same form, and twenty-four hours afterwards legitimately with pollen from a short- styled dark-red polyanthus which is a variety of P. veris; and the result was that every one of the thirty seedlings thus raised bore flowers more or less red, showing plainly how prepotent the legitimate pollen from a short-styled plant was over the illegitimate pollen from a long-styled plant.
In all the several foregoing points the parallelism is wonderfully close between the effects of illegitimate and hybrid fertilisation. It is hardly an exaggeration to assert that seedlings from an illegitimately fertilised heterostyled plant are hybrids formed within the limits of one and the same species. This conclusion is important, for we thus learn that the difficulty in sexually uniting two organic forms and the sterility of their offspring, afford no sure criterion of so-called specific distinctness. If any one were to cross two varieties of the same form of Lythrum or Primula for the sake of ascertaining whether they were specifically distinct, and he found that they could be united only with some difficulty, that their offspring were extremely sterile, and that the parents and their offspring resembled in a whole series of relations crossed species and their hybrid offspring, he might maintain that his varieties had been proved to be good and true species; but he would be completely deceived. In the second place, as the forms of the same trimorphic or dimorphic heterostyled species are obviously identical in general structure, with the exception of the reproductive organs, and as they are identical in general constitution (for they live under precisely the same conditions), the sterility of their illegitimate unions and that of their illegitimate offspring, must depend exclusively on the nature of the sexual elements and on their incompatibility for uniting in a particular manner. And as we have just seen that distinct species when crossed resemble in a whole series of relations the forms of the same species when illegitimately united, we are led to conclude that the sterility of the former must likewise depend exclusively on the incompatible nature of their sexual elements, and not on any general difference in constitution or structure. We are, indeed, led to this same conclusion by the impossibility of detecting any differences sufficient to account for certain species crossing with the greatest ease, whilst other closely allied species cannot be crossed, or can be crossed only with extreme difficulty. We are led to this conclusion still more forcibly by considering the great difference which often exists in the facility of crossing reciprocally the same two species; for it is manifest in this case that the result must depend on the nature of the sexual elements, the male element of the one species acting freely on the female element of the other, but not so in a reversed direction. And now we see that this same conclusion is independently and strongly fortified by the consideration of the illegitimate unions of trimorphic and dimorphic heterostyled plants. In so complex and obscure a subject as hybridism it is no slight gain to arrive at a definite conclusion, namely, that we must look exclusively to functional differences in the sexual elements, as the cause of the sterility of species when first crossed and of their hybrid offspring. It was this consideration which led me to make the many observations recorded in this chapter, and which in my opinion make them worthy of publication.
CHAPTER VI. CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants. Summary of the differences in fertility between legitimately and illegitimately fertilised plants. Diameter of the pollen-grains, size of anthers and structure of stigma in the different forms. Affinities of the genera which include heterostyled species. Nature of the advantages derived from heterostylism. The means by which plants became heterostyled. Transmission of form. Equal-styled varieties of heterostyled plants. Final remarks.
In the foregoing chapters all the heterostyled plants known to me have been more or less fully described. Several other cases have been indicated, especially by Professor Asa Gray and Kuhn, in which the individuals of the same species differ in the length of their stamens and pistils (6/1. Asa Gray 'American Journal of Science' 1865 page 101 and elsewhere as already referred to. Kuhn 'Botanische Zeitung' 1867 page 67.); but as I have been often deceived by this character taken alone, it seems to me the more prudent course not to rank any species as heterostyled, unless we have evidence of more important differences between the forms, as in the diameter of the pollen-grains, or in the structure of the stigma. The individuals of many ordinary hermaphrodite plants habitually fertilise one another, owing to their male and female organs being mature at different periods, or to the structure of the parts, or to self-sterility, etc.; and so it is with many hermaphrodite animals, for instance, land-snails or earth-worms; but in all these cases any one individual can fully fertilise or be fertilised by any other individual of the same species. This is not so with heterostyled plants: a long-styled, mid-styled or short-styled plant cannot fully fertilise or be fertilised by any other individual, but only by one belonging to another form. Thus the essential character of plants belonging to the heterostyled class is that the individuals are divided into two or three bodies, like the males and females of dioecious plants or of the higher animals, which exist in approximately equal numbers and are adapted for reciprocal fertilisation. The existence, therefore, of two or three bodies of individuals, differing from one another in the above more important characteristics, offers by itself good evidence that the species is heterostyled. But absolutely conclusive evidence can be derived only from experiments, and by finding that pollen must be applied from the one form to the other in order to ensure complete fertility.
In order to show how much more fertile each form is when legitimately fertilised with pollen from the other form (or in the case of trimorphic species, with the proper pollen from one of the two other forms) than when illegitimately fertilised with its own-form pollen, I will append Table 6.33 giving a summary of the results in all the cases hitherto ascertained. The fertility of the unions may be judged by two standards, namely, by the proportion of flowers which, when fertilised in the two methods, yield capsules, and by the average number of seeds per capsule. When there is a dash in the left hand column opposite to the name of the species, the proportion of the flowers which yielded capsules was not recorded.
TABLE 6.33. Fertility of the legitimate unions taken together, compared with that of the illegitimate unions together. The fertility of the legitimate unions, as judged by both standards, is taken as 100.
Column 1: Name of species. Column 2: Illegitimate unions : proportional number of flowers which produced capsules. Column 3: Illegitimate unions : average number of seeds per capsule.
Primula veris : 69 : 65.
Primula elatior : 27 : 75.
Primula vulgaris : 60 : 54.
Primula Sinensis : 84 : 63.
Primula Sinensis (second trial) : 0 : 53.
Primula Sinensis (Hildebrand) : 100 : 42.
Primula auricula (Scott) : 80 : 15.
Primula Sikkimensis (Scott) : 95 : 31.
Primula cortusoides (Scott) : 74 : 66.
Primula involucrata (Scott) : 72 : 48.
Primula farinosa (Scott) : 71 : 44.
Average of the nine species of Primula : 88.4 : 69.
Hottonia palustris (H. Muller) : - : 61.
Linum grandiflorum (the difference probably is much greater) : - : 69.
Linum perenne : - : 20.
Linum
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