4 i6 



SPHENOPHYLLALES. B. PSILOTACEAE 



is attained. But the tissue of the normal septum is of common origin 

 with the sporogenous tissue itself, and in the earliest stages does not differ 

 from it in position or structure. The normal development, in fact, would 

 be compatible with the view that the septum is formed from potential 

 sporogenous tissue sterilised : the unilocular condition would then follow 

 from reversion of that sterile tissue to its original fertile state again. 

 This question is intimately connected with that of the origin of the 

 sporangiophore, and opportunity will be taken later to discuss it. 



The detailed study of the sporangiophore or synangium of Psilotitm by 

 means of sections is more difficult than that of Tmesipteris on account of 

 its trilocular character. In radial sections through the terminal bud the 





- . k 



' 





A 



FIG. 231. 



'- 



B 



Tmesipteris tantiiiisis. Bernh. A, median section through synangium, showing the 

 tissue where the septum normally is developing as sporogenous cells (s). / = tapetum. 

 B, part of the contents of a similar synangium, rather older, jc.r shows the line where 

 the septum should normally be, while a chain of fertile cells stretches continuously across 

 it. This drawing also indicates how cells distributed through the sporogenous tissue 

 become disorganised. X 100. 



young sporophylls are found to present a general outline and structure 

 similar to those of Tmesipteris. Fig. 232 A shows one such : the cell ( x ) 

 is believed to represent the organic apex of the sporophyll, though it is 

 doubtful whether it be this initial which gives rise to the whole mass of 

 the tissue. The sporangiophore appears as an outgrowth of the upper 

 surface of the sporophyll, while the tissue on the abaxial side of it is 

 already growing out into a bulky projection. But it has not been possible 

 to trace the development of the essential parts of the loculi of the 

 synangium from the superficial cells of the adaxial side of the sporophyll 

 in this case with the same certainty as in Tmesipteris : this is chiefly 

 owing to the stalk being here narrower, and to the fact that only one 

 loculus can be cut in a median direction in any one section ; supposing 

 this to be the median plane of the whole sporophyll, then it will be the 



