PHANEROGAMS. 



423 



rogams, the Seed, the testa of which, the product of the envelopes of the ovule, 

 closely invests both endosperm and embryo. The whole becomes separated from 

 the mother-plant after the embryo has attained a certain very variable degree of 

 development. Germination consists in the further development of the embryo at 

 the expense of the endosperm. 



If, on the other hand, the microspores of Selaginella and Isoetes are compared 

 with the pollen-grains of Phanerogams, a series of analogies is again seen which be- 

 comes intelligible on comparing the intermediate phenomena presented by Gymno- 

 sperms. Indications of the male prothallium and antheridium are indicated, as Millardet 

 and Pfeffer have shown, by certain cell-divisions which may also be recognised in a 

 still simpler form in the pollen-grain of Gymnosperms, but which do not occur in 

 Angiosperms. Like the microspores, the pollen-grains contain the male fertilising 

 principle, which, passing into the oosphere or embryonic vesicle, causes it to develope 

 the embryo ; but a great difference is displayed in the mode in which the fertilising 

 substance is conveyed. In Cryptogams the fertilising substance takes the form of 

 spermatozoids or antherozoids endowed with motion and adapted to force them- 

 selves, with the assistance of water, into the oosphere through the open neck of the 

 archegonium. In Phanerogams, where the embryonic vesicle is enclosed in the 

 embryo-sac and ovule, and in Angiosperms is also surrounded by the wall of the 

 ovary, such a conveyance of the fertilising element would not serve the purpose 

 intended; the pollen-grains are therefore themselves conveyed to the female organ 

 by foreign agencies, such as the wind, mechanical contrivances in the flowers, and 

 especially insects; and then germinating like spores, they emit their pollen-tubes, 

 which, penetrating through the masses of tissue of the female organ, finally reach 

 the embryo-sac, and transmit by diffusion the amorphous soluble fertilising substance 

 into the embryonic vesicle. The analogy of pollen-grains to spores becomes still 

 more evident when we examine the mode of origin of both. The mass of tissue 

 in which the pollen is formed, the pollen-sac, shows, not only in its morphological 

 but also in its anatomical relationships, a striking resemblance to the sporangium of. 

 Vascular Cryptogams. As in the latter the spore-mother-cells are formed by the 

 isolation of cells previously combined, so also are the mother-cells of the pollen; 

 and as the former themselves produce the spores by division into four, usually 

 after previous indication of a bipartition, the pollen-cells are produced from their 

 mother-cells in a similar manner. Moreover, in the points here indicated Gymno- 

 sperms again appear as a connecting link between Cryptogams and Angiosperms ; 

 the pollen-sacs of Cycadeae and of some Coniferae resembling, in form and position, 

 the sporangia of some Vascular Cryptogams. 



The general result of these observations is that the Phanerogam, with its 

 pollen-grains and its embryo-sac, is equivalent to the spore-producing (asexual) 

 generation of the heterosporous Vascular Cryptogams. But as in Vascular Cryp- 

 togams the sexual differentiation first makes its appearance (in Ferns and Equise- 

 tace«) on the prothallium only, and next (in Rhizocarpeae and Lycopodiaceae) on 

 the spores themselves, so, in Phanerogams, this process is carried back a step 

 further, the sexual differentiation arises still earlier, being manifested not only in 

 the formation of embryo-sac and pollen-grains, but also in the difference between 

 ovule and pollen-sac, and even earlier in the distinction between male and 



