VASCULAR CRYPTOGAMS 15 



e embryo-sac, the homologiie of the megaspore. The nucellus of the 

 'ule must then be regarded as corresponding to the megasporange ; 

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

 Ddiacese an hermaphrodite prothallium usually exposes both kinds of 

 ixual organ to the action of moisture ; in Equisetacese we find a normal 

 fferentiation into male and female prothallia, but produced from one 

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

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

 altogether a subordinate product, and never has any separate existence 

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

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

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

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

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

 n the Selaginellacese we find also the rudiments of two other structures 

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

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

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

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

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

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

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

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

 Cryptogams. 



In all the isosporous Vascular Cryptogams the sexual generation or 

 ophyte has an independent existence distinct from the spore which 

 reduced it ; while in the heterosporous families the prothallium recedes 

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

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

 lentary in structure. In Gymnosperms the female prothallium and the 

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

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

 ^haracese alternation of generations disappears by the suppression of 

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

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

 iression of the sexual generation or prothallium with its antherids and 

 rchegones, and the coalescence of male and female elements takes 

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

 orresponds to the megaspore. 



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

 /■ascular Cryptogams may now be described more in (Jetail. 



The immediate product of the germination of the spore or megaspore 



