GROUP III. PTERIDOPHYTA : FILICIN^ ; LEPTOSPORANGIAT. 399 



of tissue filling the venter of the archegonium : it absorbs from the 

 adjacent cells the organic substances formed in the prothallium by 

 means of the chloroplastids which most of the cells contain. The 

 primary root and the cotyledon are both small and short-lived : 

 the former is succeeded by the numerous adventitious roots, the 

 latter by the true foliage-leaves. The foot is a merely embryonic 

 organ : it disappears when the young sporophy te has become firmly 

 attached to the substratum, and is capable of independently 

 absorbing and assimilating food. 



GAMETOPHYTE. The gametophyte is a prothallium, always con- 

 taining chloroplastids, generally a dorsi ventral, flattened, cellular 

 expansion, or sometimes filamentous, which is developed from a 

 spore, but which becomes completely free from the spore ; there 

 is frequently a more or less marked differentiation of a sexual 

 receptacle, which may attain the dignity of a gametophore, but is, 

 however, exclusively female. In the dorsiventral prothallium the 

 reproductive organs, as also the root- hairs, are confined to the in- 

 ferior (ventral) surface. 



The prothallium is typically monoecious : 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 

 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 ; the prothallium is in effect dichogamous, 

 arid, consequently, cross-fertilisation is almost certainly ensured. 



The development of the prothallium commences with the rupture 

 of the outer coat (exospore) of the germinating spore, which takes 

 place either along three lines meeting at an angle, when the spore 

 is tetrahedral, or by a longitudinal slit when the spore is bilateral, 

 the contents covered by the inner coat (endospore) being exposed. 

 Most commonly this cell grows out into a filament, cell-divisions 

 taking place in the transverse plane only, so that the prothallium 

 consists of a longitudinal row of cells. At length a longitudinal 

 wall is formed in the terminal cell of the filament ; cell-division 



