THE PTERIDOPHYTA : LYCOPSIDA, ETC. 613 



layer extend into two-celled rhizoids. In starved conditions the upper cell 

 of many of the rhizoids divides and produces a small nodular gemma, 

 which on being detached develops into a new rhizome. 



At the extreme base of the stem the stele is protostelic, as in the rhizome, 

 but the xylem is often surrounded by scattered secondary tracheids, apparently 

 relics of ancestral secondary thickening. No cambium is, however, formed. 

 At this level the stele resembles that of the fossil SpJienophylhim, but a little 

 higher up a parenchymatous pith appears, which still higher gives place to 



#« 



• 



• • , 





,1 i 



Bract 



Fig. 624. — Psilotum triquetrnm. Longi- 

 tudinal section of a synangium showing 

 two groups of archesporium and 

 sporangiophore. 



the typical central sclerenchyma, and thus the resemblance is lost in the 

 mature stem. 



Development of the Sporangium 



It has been for some time an open question whether we had here to deal 

 with a true synangium, of united sporangia, or with a single trilocular 

 sporangium. The earliest stages of development show, however, that each 

 loculus arises separately from a single epidermal cell of the sporangiophore. 

 We may therefore assume that the structure is a union of three sporangia, 

 that is, a true synangium. The first division of the primary cell is periclinal 

 and separates a jacket cell, from which a wall three to five cells thick develops, 

 and one archesporial cell which produces a central mass of sporogenous 

 cells (Fig. 624). The three sporangia are grouped around a central sterile 



