CRYPTOGAMS 



397 



a dome-shaped columella, which in turn is overarched by a hemispherical spore-sac 

 {spo) ; it possesses an operculum, but no peristome. The ripe sporogonium, like 

 that of Andreaea, is borne upon a prolongation of the stem axis, the pseudopodium, 

 which is expanded at the top to receive the foot of the stalk. Of the peculiar 

 structure of the leaves and stem cortex a description has already been given (p. 390). 



III. PTERIDOPHYTA (VASCULAR CRYPTOGAMS) 



The Pteridophytes include the Ferns, Water-Ferns, Horse-tails, 

 and Club Mosses, and represent the most highly developed Cryptogams. 

 In the development of the plants forming this group, as in the Bryo- 

 phyta, a distinct alternation of generations is exhibited. The first 

 generation, the sexual, bears the antheridia and archegonia ; the 

 second, the asexual, develops from the fertilised egg and produces 

 asexual, unicellular spores. On germination the spores in turn give rise 

 to a sexual generation. Both the sexual and asexual generations 

 of the Pteridophyta present marked variations in the mode of their 

 development. 



The sexual generation is termed the prothallium or game- 

 tophyte. In some forms it never reaches anj r great size, being at most 

 a few centimetres in diameter ; it resembles in appearance a simple, 

 thalloid Liverwort ; it 



then consists of a small 

 green thallus, attached 

 to the soil by rhizoids 

 springing from the under 

 side (Fig. 330, A). At 

 other times the prothal- 

 lium is branched and 

 filamentous ; sometimes it 

 is a tuberous, colourless 

 mass of tissue, and parti- 

 ally or wholly buried in 

 the ground, leading a 

 saprophytic existence, 

 while in certain other 

 divisions of the Pterido- 

 phyta it undergoes reduc- 

 tion and remains more or 

 less completely enclosed 

 within the spore. On the prothallia arise the sexual organs, anther- 

 idia producing numerous ciliate, usually spiral spermatozoids, and 

 archegonia, in each of which is a single egg-cell. As in the Mosses 

 the presence of water is necessary for fertilisation. 



After fertilisation the egg-cell develops into a multicellular embryo, 



Fig. 330. — Aspidivm filix mas. A, Prothallium seen from 

 below ; ar, archegonia ; an, antheridia ; rh, rhizoids ; B, 

 prothallium with young Fern attached to it by its foot ; 

 b, the first leaf ; w, the primary root, (x circa 8.) 



