On the other hand, the anthers of the short-styled are usually longer and contain larger pollen grains than those of the long-styled, the pollen of which is also often more translucent and smoother.

Of all the species included in the above-mentioned thirtysix species, only five seem to have the pollen of both forms of the same size, and two in which it was reversed. The five species are Leucosmia Burnettiana, Linum grandiflorum, Cordia, Gilia pulchella, and Coccocypselum. The two in which the pollen grains of the long-styled form were the larger, were Gilia micrantha and Phlox subulata.

The presence of cases where the usual differences are not pronounced is just what one expects to find, in accordance with the laws of differentiation; whereby intermediate conditions are to be looked for. Thus some species of Primula afford great differences in the shapes of the stigmas, P. veris being globular in the long-styled, and depressed in the short-styled; while in P. Sinensis it is elongated: but in other species, as P. Sikkimensis and P. farinosa, there is but little difference between the stigmas of the two forms. In some cases the differences reside entirely in the stamens or pollen grains, as in Forsythia suspensa, in which, although (contrary to the rule) the anthers of the long-styled are in length as 100: 87 compared with the short-styled, yet the pollen grains are as 94: 100, which agrees with the rule. With Linum grandiflorum and Cordia and Gilia pulchella, etc., the difference lies in the pistil. On the other hand, the difference may reside in the stamens, as in Egiphila elata, the pollen grains being as 62: 100, i.e. in the long-styled as compared with the short-styled.

Egiphila obdurata has the stigmas of the long-styled in length 100:55 as compared with the short-styled; and the length of the anthers as 44: 100. This is, therefore,

apparently truly heterostyled, but from Mr. Darwin's observations he thinks the short-styled incapable of fertilisation; moreover the anthers of the long-styled form were “brown, tough, and devoid of pollen." He considers that, from having been heterostyled, it has now become diœcious, or else gynodioecious.

M. W. Burck has shown * that several genera of Rubiacea are heterostyled in form but quite diœcious.

Faramea affords another curious difference. In the longstyled form the stigma is short and broad; in the shortstyled, it is long, thin, and curled. The anthers of the short-styled are a little larger than those of the long-styled, and the size of their pollen grains are as 100: 67. But the most remarkable difference (of which no other instance is known) is in the fact that while the pollen grains of the short-styled forms are covered with sharp points, the smaller ones are quite smooth. The anthers, moreover, rotate outwards in the short-styled, but do not do so in the long-styled flowers. A similar rotation takes place in some of the Cruciferæ, and facilitates intercrossing. A somewhat analogous torsion occurs in some styles and stigmas, as of Linum perenne, Luzula arvensis, Begonia, etc.

The smaller and smooth pollen, in the more degenerate condition of the long-styled form, is suggestive of the origin of that of wind-fertilised flowers, which has sometimes acquired the same form. Indeed, the two forms of pollen (figured by Mr. Darwin at p. 129 of Forms of Flowers) exactly correspond to the very common spinescent form in intercrossing species of Composite, and to that of the anemophilous Artemisia of the same order, respectively.

The general conclusion, therefore, derived from the com

* Sur l'Organisation Florale chez quelques Rubiacées. Ann. Jard. Bot. Buitenzorg 3, p. 105.

:

parison of these minute details, is that the long-styled form of flower represents a more fully developed pistil, and therefore a more female condition; while the short-styled is more male and, as we have seen above, this is borne out by the comparison of the offspring; and, lastly, by the probable diœcious condition of Egiphila obdurata, as well as by the actual diœcism of some species of Mussanda and Morinda umbellata; while Mussanda cylindrocarpa and certain other species of Morinda are hermaphrodite without heterostylism (Burck, l.c.).

CHAPTER XXIV.

PARTIAL DICLINISM.

GYNODIECISM AND GYNOMONECISM. *-In accounting for the origin of certain floral structures, it must be borne in mind that the habits and constitutions of plants are so infinitely various, that the interpretation given for that of a structure in one case may fail to be satisfactory when tested by another; and an argument apparently sound for the explanation of a special phenomenon in a particular plant or plants may not at all apply to that of others. Thus, while the Hazel may mature its stamens before the pistils on a slight rise of temperature in early spring, there are many herbs, if they happen to blossom in spring earlier than is their custom, in summer, or what may be their optimum period, may have the staminal whorl more or less deranged, as such plants require a relatively higher temperature to develop them perfectly. This is particularly characteristic of gynodioecious plants. Thus, e.g., most of the distinctly protandrous species of the Alsineæ are in this condition, and

* Gynodiæcism signifies that the same species may have both female and hermaphrodite plants.

Gynomonacism signifies that the same plant may bear both female and hermaphrodite flowers.

This will be discussed more fully in the next chapter.

the plants with small, usually pistillate flowers are chiefly in blossom at the beginning of the flowering period of the larger-flowered hermaphrodite plants of this section of the Caryophylleæ. Similarly, Caffea arabica produces small pistillate flowers in Guatemala at the beginning of the season. ** It is the same with Geranium macrorhizon and many species of Pelargonium, etc. †

Gynodiccism also prevails in the Labiata, but both female and hermaphrodite plants for the most part blossom simultaneously in summer. It may be noticed that the corolla is almost invariably reduced in size in female flowers, whether the species be strictly diœcious as in Bryony, or gynodiœcious as Thyme, showing the close interdependence between the corolla and stamens. ‡

That climatal conditions are likewise connected with the Gynodiccism of the Labiata seems probable from the behaviour of Thymus Serpyllum; for Delpino found that it was trimorphic in the warmer region of Florence, having flowers with greatly developed stamens and the pistil in every stage of abortion or even absent (see Chapter XXV.); other flowers showed the exact converse; and, lastly, others were hermaphrodite. Müller, however, on the other hand, in Westphalia and Thuringia; Ascherson, in Brandenburg; Hildebrand, in the Rhine provinces; and Mr. Darwin, in England, never met with the purely male form; though Dr. Ogle found some with the pistil permanently immature.§ Similarly, Eriophorum angustifolium is gynodiœcious in Scotland and the Arctic regions.||

Besides temperature, the character of the soil has most probably much effect in bringing about this kind of partial

*Müller, Fertilisation, etc., p. 304.

See Forms of Flowers, pp. 307-309. § Müller, l.c., p. 474.

+L.c., p. 158.

Forms of Flowers, p. 307.