CRYPTOGAMS. 75 



The most pronounced distinctions between the two 

 divisions, as at present understood, are as follows — 



(i) In Cryptogams the sexually produced reproductive 

 body never enters into a resting-stage after undergoing 

 cell-division and differentiation. In the lower forms, as illus- 

 trated by Spirogyra, the zygospore clothes itself with a firm 

 cell-wall, and passes the winter in a passive condition as a 

 unicellular body, germinating in the spring. In the higher 

 groups the fertilized oosphere — the oospore — at once se- 

 cretes a cell-wall of cellulose, and then divides into a number 

 of undifferentiated cells. In this state it is known as the 

 embryo. The earliest subsequent division of the embryo is 

 into the rudiments of the first root, first leaf or cotyledon, 

 growing-point or stem, and foot or organ which furnishes the 

 plantlet with its first food, absorbed from the prothallus. 

 The point to be remembered is, that when this stage of 

 differentiation has been reached there is no resting-stage ; the 

 structure must uninterruptedly continue its growth or perish. 

 In Phanerogams the fertilized oosphere by cell-division 

 becomes differentiated into a minute plantlet or embryo, 

 presenting a root, stem, and one or two leaves or cotyledons. 

 This development takes place while the embryo is yet 

 surrounded by the coverings of the ovule, which become 

 converted into an external protective covering, the whole 

 constituting a seed, and at this period corresponding to the 

 maturity of the seed, the entire structure enters the resting 

 state. The advantage of a resting-stage, as affecting the 

 dispersion of species, is obvious, and indeed becomes indis- 

 pensable, inasmuch as the asexual mode of reproduction by 

 spores, which facilitated dispersion in Cryptogams, is arrested 

 in Phanerogams. 



(2) Alternation of generations is very clearly marked in 



