VASCULAR CRYPTOGAMS 15 



the embryo-sac, the homologue of the me'gaspore. The nucellus of the 

 ovule must then be regarded as corresponding to the megasporange ; 

 but it is difficult to carry the homology further. In Filices and Lyco- 

 podiaceae an hermaphrodite prothallium usually exposes both kinds of 

 sexual organ to the action of moisture ; in Equisetaceae we find a normal 

 differentiation into male and female prothallia, but produced from one 

 kind of spore only ; in the heterosporous families the differentiation is 

 carried back to the spores and sporanges, and the female prothallium 

 is altogether a subordinate product, and never has any separate existence 

 apart from the megaspore, within which it is more or less concealed, 

 only that part which bears the archegones being exposed. In Angio- 

 sperms it has been suggested that we have a rudiment of the female 

 prothallium surviving in the peculiar ' antipodal cells ' found within the 

 embryo-sac in certain natural orders ; but the homology is doubtful. 

 In the Selaginellaceae we find also the rudiments of two other structures 

 which characterise the ovule of Flowering Plants. The sterile tissue which 

 occupies the lower part of the megaspore in this order is probably the 

 first appearance of the endosperm (albumen) which is found in the seed 

 of a large number of Flowering Plants ; the purpose in both cases being 

 the same, to provide the embryo with nutritive material during the early 

 stages of its growth. In all Phanerogams the young embryo is borne 

 on a longer or shorter pedicel of cellular structure, the suspensor or pro- 

 embryo, which, again, we find for the first time in the same order of 

 Cryptogams. 



In all the isosporous Vascular Cryptogams the sexual generation or 

 oophyte has an independent existence distinct from the spore which 

 produced it ; while in the heterosporous families the prothallium recedes 

 more and more into the background, existing only within the megaspore ; 

 and at the same time the male organs or antherids become more rudi- 

 mentary in structure. In Gymnosperms the female prothallium and the 

 antherids (as distinct from the antherozoids) have become much reduced, 

 and in Angiosperms have completely disappeared. While, therefore, in 

 Characeae alternation of generations disappears by the suppression of 

 the non-sexual generation which bears the spores, in Phanerogams the 

 same result is brought about by an exactly opposite process, the sup- 

 pression of the sexual generation or prothallium with its antherids and 

 archegones, and the coalescence of male and female elements takes 

 place within the non-sexually produced embryo-sac of the ovule, which 

 corresponds to the megaspore. 



The life-history and general structure of the various organs in 

 Vascular Cryptogams may now be described more in detail. 



The immediate product of the germination of the spore or megaspore 



