294 



MOSSES AND FERNS 



CHAP. 



elongated elevation composed of cells which have divided by 

 periclinal walls. In /. lacustris the sporangium arises mainly 

 from the three outer layers of cells thus formed. The lower 

 part of this cushion-shaped prominence forms the base or stalk, 

 while the archesporium is form.ed from the hypodermal layer of 

 cells (Fig. 155, A). Each cell of the archesporium shows an 

 independent growth, and up to this point the development of 

 macro- and micro-sporangia is the same. In the latter each 



B. 



Fig. 153. — Isoetes laaistris (L.). A, Longitudinal section of young microsporangium ; B, similar 

 section of macrosporangium. The shaded cells in A, the nucleated ones in B, represent the 

 archesporium (after Goebel) ; C, transverse section of the sporophyll and microsporangium, 

 X 8 ; tr, the trabeculse (after Bower). 



archesporial cell divides by a series of tangential walls and 

 at first all appear alike ; but soon some of the rows become 

 more transparent and divide less rapidly, so that they form 

 elongated tabular cells. The others divide in all directions 

 and form large masses of cells with abundant protoplasm. 

 These finally form the spore mother cells. The outer cells of 

 both sporogenous and sterile rows form the tapetum. The 

 sporangium at this stage consists of a series of irregular 

 chambers separated by incomplete layers of colourless cells 



