_Proterogynous_, when the stigmas are mature and moistened for the reception of pollen, before the anthers of that blossom are ready to supply it, and are withered before that pollen can be supplied. Plantains or Ribworts (mostly wind-fertilized) are strikingly proterogynous: so is Amorpha, our Papaws, Scrophularia, and in a less degree the blossom of Pears, Hawthorns, and Horse-chestnut. 340. In Sabbatia, the large-flowered species of Epilobium, and strikingly in Clerodendron, the dichogamy is supplemented and perfected by movements of the stamens and style, one or both, adjusted to make sure of cross fertilization. 341. =Heterogony.= This is the case in which hermaphrodite and fertile flowers of two sorts are produced on different individuals of the same species; one sort having higher anthers and lower stigmas, the other having higher stigmas and lower anthers. Thus reciprocally disposed, a visiting insect carries pollen from the high anthers of the one to the high stigma of the other, and from the low anthers of the one to the low stigma of the other. These plants are practically as if diœcious, with the advantage that both kinds are fruitful. Houstonia and Mitchella, or Partridge-berry, are excellent and familiar examples. These are cases of _Heterogone Dimorphism_, the relative lengths being only short and long reciprocally. _Heterogone Trimorphism_, in which there is a mid-length as well as a long and a short set of stamens and style; occurs in Lythrum Salicaria and some species of Oxalis. 342. There must be some essential advantage in cross fertilization or cross breeding. Otherwise all these various, elaborate, and exquisitely adjusted adaptations would be aimless. Doubtless the advantage is the same as that which is realized in all the higher animals by the distinction of sexes. § 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO. 343. =Pollen-growth.= A grain of pollen may be justly likened to one of the simple bodies (_spores_) which answer for seeds in Cryptogamous plants. Like one of these, it is capable of germination. When deposited upon the moist surface of the stigma (or in some cases even when at a certain distance) it grows from some point, its living inner coat breaking through the inert outer coat, and protruding in the form of a delicate tube. This as it lengthens penetrates the loose tissue of the stigma and of a loose conducting tissue in the style, feeds upon the nourishing liquid matter there provided, reaches the cavity of the ovary, enters the orifice of an ovule, and attaches its extremity to a sac, or the lining of a definite cavity, in the ovule, called the _Embryo-Sac_. 344. =Origination of the Embryo.= A globule of living matter in the embryo-sac is formed, and is in some way placed in close proximity to the apex of the pollen tube; it probably absorbs the contents of the latter; it then sets up a special growth, and the _Embryo_ (8-10) or rudimentary plantlet in the seed is the result. FOOTNOTES: