INTR ODUCriON. 487 



representative of the macrospore, that part of the ovule in which the embryo-sac 

 arises (the nucellus) must be considered the equivalent of the macrosporangium. 

 But, as in the formation of the ovules of Monocotyledons and Dicotyledons, certain 

 processes of development (the formation of the archegonia or ' corpuscula '), being 

 no longer necessary, are suppressed, and the oosphere is immediately produced 

 within the embryo-sac as the analogue of the macrospore, so also the production 

 of the embryo-sac immediately from the tissue of the nucellus of the ovule is more 

 direct. Its production is due to the increase in size of an inner cell of the nucellus 

 which here represents the sporangium. But while even in the most highly developed 

 Cryptogams the macrospore still becomes detached from the mother-plant, and the 

 full development of the prothallium takes place only after the dissemination of the 

 spores, so that the embryo always arises in structures distinct from those of the 

 mother-plant, the embryo-sac (or macrospore) of all Phanerogams remains, on 

 the contrary, enclosed in the ovule, the endosperm in the embryo- sac, and the 

 embryo in the endosperm. In this manner arises that structure peculiar to Phane- 

 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 Selagiftella 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 recog- 

 nised in a simpler form in the pollen-grain of Gymnosperms and in a still simpler 

 form in those of Angiosperms. Like the microspores, the pollen-grains contain the 

 male fertilising substance, which, passing into the oosphere, 'causes it to develope the 

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

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

 antherozoids endowed with motion and adapted to force themselves, with the 

 assistance of water, into the oosphere through the open neck of the archegonium. 

 In Phanerogams, where the oosphere is enclosed in the embryo-sac and ovule, and 

 in Angiosperms by the wall of the ovary in addition, such a conveyance of the 

 fertilising substance would not serve the purpose intended; the pollen-grains are 

 therefore themselves conveyed to the ovule 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 tissue of the 

 ovule, finally reach the embryo-sac, and transmit the fertilising substance to the 

 oosphere. 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 

 usually produce the spores by division into four, after previous indication of a 



