PTERIDOPHYTA. 



153 



PTERIDOPHYTA. 



177. The sixth division, called Pteridophyta, include 

 the Ferns (Fig. 265) and their allies. Here, as in the 

 previous division, there is an alternation of sexual and 

 non-sexual generations. But while the conspicuous gen- 

 eration (the Moss) in the Bryophyta is sexual (and the 

 inconspicuous, namely, the sporangium, non-sexual), the 

 reverse is the case in the Pterido- 

 phyta, that is, the conspicuous 

 generation (the Fern, etc.) is non- 

 sexual ; and the sexual generation, 

 or stage, bearing the sexual repro- 

 ductive organs (the prothallium) 

 is very much reduced and short- 

 lived. This prothallium is a small, 

 flattened, thallus-like growth from 

 the spore, composed of chlorophyll- 

 bearing parenchymous cells, in one 

 or a few layers ; on its under sur- 

 face are produced rhizoids, by which 

 it is fixed to the ground. On the 

 prothallia are developed the arche- 

 gonia and antheridia, which are 

 essentially similar to those in the higher plants of the pre- 

 ceding division. The spirally-coiled spermatozoids escape 

 from the antheridium, enter the tube, or neck, of the 

 archegonium, and fertilize the germ-cell therein con- 

 tained. The result of this is the formation of a young 

 plantlet, which develops into a leafy planl of considerable 

 size, with marked differentiation of tissue, and capable of 

 producing non-sexual spores. 



Fig. 265. A Fern {Cantpiosorus rhizopkyllus) ; /r , frond; sor, son; i«, indu- 

 sium ; spn, sporangium ; spo^ spores. 



