74 



OPHIOGLOSSACEAE 



[CH. 



such cell can be found (Fig. 373, b, c). Fig. 373, b, shows the most regular 

 type of these very variable organs. Already in the centre of it longitudinal 

 divisions are taking place to form the vascular strand, but in such early 

 stages it is impossible to distinguish the cells which will give rise to the 

 sporangia. Slightly older stages show that the sporogenous group of each 

 sporangium together with the superficial cells that cover it are referable to 

 the segmentation of a single superficial cell (Fig. 374, e,f): the first periclinal 

 wall defines the whole of the sporogenous tissue from the protective wall. 

 As the sporangia grow older they project from the surface: the sporogenous 



Fig. 374. Helmiiithostachys: a, transverse section of a young spike, near to 

 the base of the fertile region, enlarged, showing sporangiophores {s)\ ^, section 

 of an older spike, near the tip, with sporangiophores ( x 12) ; c, a single large 

 branched sporangiophore, with sporangia; d, young sporangiophores seen 

 from above, showing the variation in size ( x 24) ; e, sporangiophore with one 

 young sporangium ( x 200) ; /, an older sporangium ( x 200). 



mass increases rapidly in bulk, while the cells surrounding it, to the extent 

 of several layers, assume the character of tapetum, which gradually becomes 

 disorganised: finally the sporogenous cells separate and divide into tetrads. 

 As the sporangia approach maturity the upper part of each sporangiophore 

 may grow out into an irregular rosette of laciniae of vegetative tissue, but 

 these are not shown in the young stages here figured (compare Fig. 364, G). 

 It would be possible to arrange the genera of Ophioglossaceae in sequence 

 from those with relatively large and indefinite sporangia to those having them 

 smaller and more definite. A parallel sequence emerges also from a com- 



