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 palustris 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. 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.