GROUP III. PTERIDOPHYTA : FILICIX.E. 



367 



gonium) give rise to the growing-point of the first leaf or cotyledon : 

 of the two deeper (towards the venter of archegonium) octants, 

 the one constitutes the growing-point of the stem, whilst the 

 other gives rise to nothing beyond possibly some hairs. Of the 

 four hypobasal octants, one of the two apical octants gives rise to 

 the growing-point of the primary root, which is diametrically 

 opposite to the growing- point of the stem; whilst the other gives 

 rise to no special member : the two deeper hypobasal octants give 

 rise to the embryonic absorptive organ, the foot. The gradual 

 development of these members is dependent upon growth and 

 corresponding cell-division, and at an early stage histological 

 differentiation 

 into cortical and 

 stelar tissues is 

 apparent in 

 them. For a 

 time the tissue 

 of the venter of 

 the archegon- 

 ium keeps pace 

 by growth with 

 the increasing 

 size of the em- 

 b r y o : but 

 eventually the 

 primary root 

 and the cotyle- 

 don become free, 

 and ultimately 

 also the stem 

 (Fig. 220). In 

 the meantime 

 the embryo is 



nourished by means of the foot which has become a mass 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 chloroplajStids 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 sporophyte has become firmly attached 



FIG. 220. Section of young plant of Pteris aquilina still 

 attached to the prothallium by its foot : p prothallium ; / foot ; 

 r primary root ; s growing-point o f primary stem ; I primary 

 leaf or cotyledon. (Magnified: after Hofmeister.) 



