The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

perenne (Hildebrand) : 0 : 0.
Pulmonaria officinalis (German stock, Hildebrand) : 0 : 0.
Pulmonaria angustifolia : 35 : 32.
Mitchella repens : 20 : 47.
Borreria, Brazilian sp. : - : 0.
Polygonum fagopyrum : - : 46.
Lythrum salicaria : 33 : 46.
Oxalis Valdiviana (Hildebrand) : 2 : 34.
Oxalis Regnelli : 0 : 0.
Oxalis speciosa : 15 : 49.
The two or three forms of the same heterostyled species do not differ from one another in general habit or foliage, as sometimes, though rarely, happens with the two sexes of dioecious plants. Nor does the calyx differ, but the corolla sometimes differs slightly in shape, owing to the different position of the anthers. In Borreria the hairs within the tube of the corolla are differently situated in the long-styled and short-styled forms. In Pulmonaria there is a slight difference in the size of the corolla, and in Pontederia in its colour. In the reproductive organs the differences are much greater and more important. In the one form the stamens may be all of the same length, and in the other graduated in length, or alternately longer and shorter. The filaments may differ in colour and thickness, and are sometimes nearly thrice as long in the one form as in the other. They adhere also for very different proportional lengths to the corolla. The anthers sometimes differ much in size in the two forms. Owing to the rotation of the filaments, the anthers, when mature, dehisce towards the circumference of the flower in one form of Faramea, and towards the centre in the other form. The pollen-grains sometimes differ conspicuously in colour, and often to an extraordinary degree in diameter. They differ also somewhat in shape, and apparently in their contents, as they are unequally opaque. In the short-styled form of Faramea the pollen-grains are covered with sharp points, so as to cohere readily together or to an insect; whilst the smaller grains of the long-styled form are quite smooth.
With respect to the pistil, the style may be almost thrice as long in the one form as in the other. In Oxalis it sometimes differs in hairiness in the three forms. In Linum the pistils either diverge and pass out between the filaments, or stand nearly upright and parallel to them. The stigmas in the two forms often differ much in size and shape, and more especially in the length and thickness of their papillae; so that the surface may be rough or quite smooth. Owing to the rotation of the styles, the papillose surface of the stigma is turned outwards in one form of Linum perenne, and inwards in the other form. In flowers of the same age of Primula veris the ovules are larger in the long-styled than in the short-styled form. The seeds produced by the two or three forms often differ in number, and sometimes in size and weight; thus, five seeds from the long-styled form of Lythrum salicaria equal in weight six from the mid-styled and seven from the short-styled form. Lastly, short-styled plants of Pulmonaria officinalis bear a larger number of flowers, and these set a larger proportional number of fruit, which however yield a lower average number of seed, than the long-styled plants. With heterostyled plants we thus see in how many and in what important characters the forms of the same undoubted species often differ from one another--characters which with ordinary plants would be amply sufficient to distinguish species of the same genus.
As the pollen-grains of ordinary species belonging to the same genus generally resemble one another closely in all respects, it is worth while to show, in Table 6.34, the difference in diameter between the grains from the two or three forms of the same heterostyled species in the forty-three cases in which this was ascertained. But it should be observed that some of the following measurements are only approximately accurate, as only a few grains were measured. In several cases, also, the grains had been dried and were then soaked in water. Whenever they were of an elongated shape their longer diameters were measured. The grains from the short-styled plants are invariably larger than those from the long-styled, whenever there is any difference between them. The diameter of the former is represented in the table by the number 100.
TABLE 6.34. Relative diameter of the pollen-grains from the forms of the same heterostyled species; those from the short-styled form being represented by 100.
DIMORPHIC SPECIES.
Column 1: Name of species. Column 2: From the long-styled form : relative diameter.
Primula veris : 67.
Primula vulgaris : 71.
Primula Sinensis (Hildebrand) : 57.
Primula auricula : 71.
Hottonia palustris (H. Muller) : 61.
Hottonia palustris (self) : 64.
Linum grandiflorum : 100.
Linum perenne (diameter variable) : 100 (?).
Linum flavum : 100.
Pulmonaria officinalis : 78.
Pulmonaria angustifolia : 91.
Polygonum fagopyrum : 82.
Leucosmia Burnettiana : 99.
Aegiphila elata : 62.
Menyanthes trifoliata : 84.
Limnanthemum Indicum : 100.
Villarsia (sp.?) : 75.
Forsythia suspensa : 94.
Cordia (sp.?) : 100.
Gilia pulchella : 100.
Gilia micrantha : 81.
Sethia acuminata : 83.
Erythroxylum (sp.?) : 93.
Cratoxylon formosum : 86.
Mitchella repens, pollen-grains of the long-styled a little smaller.
Borreria (sp.?) : 92.
Faramea (sp.?) : 67.
Suteria (sp.?) (Fritz Muller) : 75.
Houstonia coerulea : 72.
Oldenlandia (sp.?) : 78.
Hedyotis (sp.?) : 88.
Coccocypselum (sp.?) (Fritz Muller) : 100.
Lipostoma (sp.?) : 80.
Cinchona micrantha : 91.
TRIMORPHIC SPECIES.
Column 1: Name of species. Column 2: Ratio expressing the extreme differences in diameter of the pollen- grains from the two sets of anthers in the three forms.
Lythrum salicaria : 60.
Nesaea verticillata : 65.
Oxalis Valdiviana (Hildebrand) : 71.
Oxalis Regnelli : 78.
Oxalis speciosa : 69.
Oxalis sensitiva : 84.
Pontederia (sp.?) : 55.
Column 1: Name of species. Column 2: Ratio between the diameters of the pollen-grains of the two sets of anthers in the same form.
Oxalis rosea, long-styled form (Hildebrand) : 83.
Oxalis compressa, short-styled form : 83.
Pontederia (sp.?) short-styled form : 87.
Pontederia other sp. mid-styled form : 86.
We here see that, with seven or eight exceptions out of the forty-three cases, the pollen-grains from one form are larger than those from the other form of the same species. The extreme difference is as 100 to 55; and we should bear in mind that in the case of spheres differing to this degree in diameter, their contents differ in the ratio of six to one. With all the species in which the grains differ in diameter, there is no exception to the rule that those from the anthers of the short-styled form, the tubes of which have to penetrate the longer pistil of the long-styled form, are larger than the grains from the other form. This curious relation led Delpino (as it formerly did me) to believe that the larger size of the grains in the short-styled flowers is connected with the greater supply of matter needed for the development of their longer tubes. (6/2. 'Sull' Opera, la Distribuzione dei Sessi nelle Piante' etc 1867 page 17.) But the case of Linum, in which the grains of the two forms are of equal size, whilst the pistil of the one is about twice as long as that of the other, made me from the first feel very doubtful with respect to this view. My doubts have since been strengthened by the cases of Limnanthemum and Coccocypselum, in which the grains are of equal size in the two forms; whilst in the former genus the pistil is nearly thrice and in the latter twice as long as in the other form. In those species in which the grains are of unequal size in the two forms, there is no close relationship between the degree of their inequality and that of their pistils. Thus in Pulmonaria officinalis and in Erythroxylum the pistil in the long-styled form is about twice the length of that in the other form, whilst in the former species the pollen-grains are as 100 to 78, and in the latter as 100 to 93 in diameter. In the two forms of Suteria the pistil differs but little in length, whilst the pollen-grains are as 100 to 75 in diameter. These cases seem to prove that the difference in size between the grains in the two forms is not determined by the length of the pistil, down which the tubes have to grow. That with plants in general there is no close relationship between the size of the pollen-grains and the length of the pistil is manifest: for instance, I found that the distended grains of Datura arborea were .00243 of an inch in diameter, and the pistil no less than 9.25 inches in length; now the pistil in the small flowers of Polygonum fagopyrum is very short, yet the larger pollen-grains from the short-styled plants had exactly the same diameter as those from the Datura, with its enormously elongated pistil.
Notwithstanding these several considerations, it is difficult quite to give up the belief that the pollen-grains from the longer stamens of heterostyled plants have become larger in order to allow of the development of longer tubes; and the foregoing opposing facts may possibly be reconciled in the following manner. The tubes are at first developed from matter contained within the grains, for they are sometimes exserted to a considerable length, before the grains have touched the stigma; but botanists believe that they afterwards draw nourishment from the conducting tissue of the pistil. It is hardly possible to doubt that this must occur in such cases as that of the Datura, in which the tubes have to grow down the whole length of the pistil, and therefore to a length equalling 3,806 times the diameter of the grains (namely, .00243 of an inch) from which they are protruded. I may here remark that I have seen the pollen-grains of a willow, immersed in a very weak solution of honey, protrude their tubes, in the course of twelve hours, to a length thirteen times as great as the diameter of the grains. Now if we suppose that the tubes in some heterostyled species are developed wholly or almost wholly from matter contained within the grains, while in other species from matter yielded by the pistil, we can see that in the former case it would be necessary that the grains of the two forms should differ in size relatively to the length of the pistil which the tubes have to penetrate, but that in the latter case it would not be necessary that the grains should thus differ. Whether this explanation can be considered satisfactory must remain at present doubtful.
There is another remarkable difference between the forms of several heterostyled species, namely in the anthers of the short-styled flowers, which contain the larger pollen-grains, being longer than those of the long-styled flowers. This is the case with Hottonia palustris in the ratio of 100 to 83. With Limnanthemum Indicum the ratio is as 100 to 70. With the allied Menyanthes the anthers of the short-styled form are a little and with Villarsia conspicuously larger than those of the long-styled. With Pulmonaria angustifolia they vary much in size, but from an average of seven measurements of each kind the ratio is as 100 to 91. In six genera of the Rubiaceae there is a similar difference, either slightly or well marked. Lastly, in the trimorphic Pontederia the ratio is 100 to 88; the anthers from the longest stamens in the short-styled form being compared with those from the shortest stamens in the long-styled form. On the other hand, there is a similar and well-marked difference in the length of the stamens in the two forms of Forsythia suspensa and of Linum flavum; but in these two cases the anthers of the short-styled flowers are shorter than those of the long-styled. The relative