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. Mitchella. Diodia. Borreria. Spermacoce.
PRIMULACEAE: Primula. Hottonia. Androsace.
OLEACEAE: Forsythia.
GENTIANACEAE: Menyanthes. Limnanthemum. Villarsia.
POLEMONIACEAE: Gilia.
CORDIEAE: Cordia.
BORAGINEAE: Pulmonaria.
VERBENACEAE: Aegiphila.
POLYGONEAE: Polygonum.
THYMELEAE: Thymelea.
MONOCOTYLEDONS.
PONTEDERIACEAE: Pontederia.
In some of these families the heterostyled condition must have been acquired at a very remote period. Thus the three closely allied genera, Menyanthes, Limnanthemum, and Villarsia, inhabit respectively Europe, India, and South America. Heterostyled species of Hedyotis are found in the temperate regions of North and the tropical regions of South America. Trimorphic species of Oxalis live on both sides of the Cordillera in South America and at the Cape of Good Hope. In these and some other cases it is not probable that each species acquired its heterostyled structure independently of its close allies. If they did not do so, the three closely connected genera of the Menyantheae and the several trimorphic species of Oxalis must have inherited their structure from a common progenitor. But an immense lapse of time will have been necessary in all such cases for the modified descendants of a common progenitor to have spread from a single centre to such widely remote and separated areas. The family of the Rubiaceae contains not far short of as many heterostyled genera as all the other thirteen families together; and hereafter no doubt other Rubiaceous genera will be found to be heterostyled, although a large majority are homostyled. Several closely allied genera in this family probably owe their heterostyled structure to descent in common; but as the genera thus characterised are distributed in no less than eight of the tribes into which this family has been divided by Bentham and Hooker, it is almost certain that several of them must have become heterostyled independently of one another. What there is in the constitution or structure of the members of this family which favours their becoming heterostyled, I cannot conjecture. Some families of considerable size, such as the Boragineae and Verbenaceae, include, as far as is at present known, only a single heterostyled genus. Polygonum also is the sole heterostyled genus in its family; and though it is a very large genus, no other species except P. fagopyrum is thus characterised. We may suspect that it has become heterostyled within a comparatively recent period, as it seems to be less strongly so in function than the species in any other genus, for both forms are capable of yielding a considerable number of spontaneously self-fertilised seeds. Polygonum in possessing only a single heterostyled species is an extreme case; but every other genus of considerable size which includes some such species likewise contains homostyled species.