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 f.a.gopyrum 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 pal.u.s.tris in the ratio of 100 to 83. With Limnanthemum Indic.u.m 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 size of the anthers was not particularly attended to in the two forms of the other heterostyled plants, but I believe that they are generally equal, as is certainly the case with those of the common primrose and cowslip.
The pistil differs in length in the two forms of every heterostyled plant, and although a similar difference is very general with the stamens, yet in the two forms of Linum grandiflorum and of Cordia they are equal. There can hardly be a doubt that the relative length of these organs is an adaptation for the safe transportal by insects of the pollen from the one form to the other. The exceptional cases in which these organs do not stand exactly on a level in the two forms may probably be explained by the manner in which the flowers are visited. With most of the species, if there is any difference in the size of the stigma of the two forms, that of the long-styled, whatever its shape may be, is larger than that of the short-styled. But here again there are some exceptions to the rule, for in the short-styled form of Leucosmia Burnettiana the stigmas are longer and much narrower than those of the long-styled; the ratio between the lengths of the stigmas in the two forms being 100 to 60. In the three Rubiaceous genera, Faramea, Houstonia and Oldenlandia, the stigmas of the short- styled form are likewise somewhat longer and narrower; and in the three forms of Oxalis sensitiva the difference is strongly marked, for if the length of the two stigmas of the long-styled pistil be taken as 100, it will be represented in the mid- and short-styled forms by the numbers 141 and 164. As in all these cases the stigmas of the short-styled pistil are seated low down within a more or less tubular corolla, it is probable that they are better fitted by being long and narrow for brushing the pollen off the inserted proboscis of an insect.
With many heterostyled plants the stigma differs in roughness in the two forms, and when this is the case there is no known exception to the rule that the papillae on the stigma of the long-styled form are longer and often thicker than those on that of the short-styled. For instance, the papillae on the long-styled stigma of Hottonia pal.u.s.tris are more than twice the length of those in the other form. This holds good even in the case of Houstonia coerulea, in which the stigmas are much shorter and stouter in the long-styled than in the short-styled form, for the papillae on the former compared with those on the latter are as 100 to 58 in length. The length of the pistil in the long-styled form of Linum grandiflorum varies much, and the stigmatic papillae vary in a corresponding manner. From this fact I inferred at first that in all cases the difference in length between the stigmatic papillae in the two forms was one merely of correlated growth; but this can hardly be the true or general explanation, as the shorter stigmas of the long-styled form of Houstonia have the longer papillae. It is a more probable view that the papillae, which render the stigma of the long-styled form of various species rough, serve to entangle effectually the large-sized pollen-grains brought by insects from the short-styled form, thus ensuring its legitimate fertilisation. This view is supported by the fact that the pollen-grains from the two forms of eight species in Table 6.34 hardly differ in diameter, and the papillae on their stigmas do not differ in length.
The species which are at present positively or almost positively known to be heterostyled belong, as shown in Table 6.35, to 38 genera, widely distributed throughout the world. These genera are included in fourteen Families, most of which are very distinct from one another, for they belong to nine of the several great Series, into which phanerogamic plants have been divided by Bentham and Hooker.
TABLE 6.35. List of genera including heterostyled species.
DICOTYLEDONS.
HYPERICINEAE: Cratoxylon.
ERYTHROXYLEAE: Erythroxylum.
Sethia.
GERANIACEAE: Linum.
Oxalis.
LYTHRACEAE: Lythrum.
Neseae.
RUBIACEAE: Cinchona.
Bouvardia.
Manettia.
Hedyotis.
Oldenlandia.
Houstonia.
Coccocypselum.
Lipostoma.
Knoxia.
Faramea.
Psychotria.
Rudgea.
Suteria.
Mitch.e.l.la.
Diodia.
Borreria.
Spermacoce.
PRIMULACEAE: Primula.
Hottonia.
Androsace.
OLEACEAE: Forsythia.