SECT. 1 



CRYPTOGAMS 



439 



The SEXUAL GENERATION is termed the PROTHALLIUM or GAMETO- 



PHYTE. It never reaches any great size, being at most a few centi- 

 metres in diameter; in 



some forms 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. 



399 A). At other times 



the prothallium is 



branched and filament- 

 ous ; sometimes it is a 



tuberous, colourless mass 



of tissue, partially or 



wholly buried in the 



ground, and 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, antheridia (Figs. 404, 411), 



producing numerous 

 ciliate, usually spiral 

 spermatozoids, and 

 archegonia (Figs. 405, 

 412), in each of which 

 is a single egg-cell. 

 As in the Mosses the 

 presence of water is 

 necessary for fertilisa- 

 tion. The spermato- 

 zoids are induced to 

 direct their motion 

 toward the archegonia 



FIG. 400. -A, rt,,-;* >,/;"//. emhry,. freed from th. archegonium, by the excretion from 



in longitudinal section (after KIENITZ-(JERLOKF) : 1, basal wall ; 

 II, transverse wall dividing the egg-cell into quadrants ; rudi- 

 ment of the foot/, of the stem s. of the lirst leaf 6, of the root 

 w ; H, section of a farther-developed embryo of Pteris aquilina 

 (after HOFMEISTER); /, foot still emlwdded in the enlarged 

 venter of the archegonium aw ; pr, prothallium. (Magnified.) 



Fie. 3'.H>. Aaiiiilniin jUiji m.. .1, I'rotlmlliiini seen from 

 lic-ln\v ; in-, archegonia ; an, antheridia ; rh, rhizoids ; B, 

 prothallium with young Fern attached to it by its foot ; 

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



the latter of a sub- 

 stance which diffuses 

 into the surrounding 

 water. In Ferns, Sal- 



vinia, Equisetum, Sela- 

 ginella, and Isoetes, this substance is malic acid or one of its salts ( 107 ). 

 After fertilisation" the egg-cell develops into a multicellular 

 embryo, which becomes the asexual generation, as in the Bryophyta. 



