582 



A TEXTBOOK OF THEORETICAL BOTANY 



RHIZOPHORE 



SPORE 



INITIAL CELL OF ROOT 



APEX OF 



RHIZOPHORE 



PRIMARY ROOT 



B 



Fig. 591. — Selaginella tnartemii. A, Young sporeling showing primary root and 

 origin of rhizophores. The small dorsal leaves are clearly shown. B, Longi- 

 tudinal section of apex of rhizophore showing endogenous origin of root initial. 

 [After Goebel.) 



mucilaginous and probably secrete water which keeps the young sporangium 

 moist (Fig. 593). 



The early development of both kinds of sporangia is identical and closely 

 follows that in Lycopodium, up to the mother-cell stage. The wall is only 

 two cells thick but there is in addition a lining of a single layer of nutritive 

 cells forming the tapetum. The colour of the spores shows through the 

 translucent wall, the microsporangia being whitish-green and the mega- 

 sporangia orange. 



In the microsporangia some of the mother cells abort, but a large number 

 develop and form tetrads of microspores, which are tetrahedral in shape, 

 cutinized, ornamented with papillae and furnished with a triradiate ridge 

 marking the line at which the spore eventually opens, or dehisces. In the 

 megasporangia all the mother cells abort except one. This produces four 

 megaspores which are nourished by the degenerating cells and the tapetal 

 fluid, and grow to such a size that they distend the sporangial wall and even 



