368 



PART IV. CLASSIFICATION". 



to the substratum, and is capable of independently absorbing and 

 assimilating food. 



GAMETOFHYTE. The gametophyte is a prothallium, always con- 

 taining chloroplastids, generally a dorsiventral, flattened, cellular 

 expansion, or sometimes filamentous, which is developed from a 

 spore, but which becomes completely free from the spore. In 

 the dorsiventral prothallium the reproductive organs, as also the 

 root-hairs, are confined to the inferior (ventral) surface. 



The prothallium is typically rnonoscious : the male organs, or 

 antheridia, are developed first, and are consequently situated 

 towards the posterior or basal end of the prothallium; the later- 

 formed archegonia lie towards the anterior or apical end. It 

 sometimes happens, however, that, owing to imperfect nutrition, 

 the growth of the prothallium does not proceed beyond the stage 



Fia. 221. Diagram of the prothal- 

 lium of a Leptosporangiate Fern : under 

 side (x 10). ar Archegonia; an anthe- 

 ridia ; fc root-hairs. 



FIG. 222. Antheridium of Adiantum 

 Capillus-Teneris (x550). p Prothallium ; 

 a antheridium ; s spermatozoid ; b the 

 vesicle containing starch-grains. 



necessary for the formation of the antheridia, so that exclusively 

 male prothallia may be sometimes found ; less commonly, well- 

 nourished prothallia fail to produce antheridia, and consequently 

 exclusively female prothallia are found. The practical result of 

 this successive formation of the antheridia and archegonia is that 

 but few of them can possibly mature at the same time on one and 

 the same prothallium ; and, consequently, cross-fertilisation is 

 almost certainly ensured. 



