338 



BOTANY OF THE LIVING PLANT 



to it by long thin stalks (Fig. 278). The head of each sporangium is 

 shaped like a biconvex lens ; its margin is almost completely sur- 

 rounded by a series of indurated cells, which form the mechanically 

 effective annulus. This stops short on one side, where several thin- 

 walled cells define the stomium, or point where dehiscence will take 



1.1. 



12, 



FIG. 279. 



Successive young stages in the segmentation of the sporangium of Nephrodium 

 Filix-mas. (After Kny.) 



place (Figs. 278, 280, 40, 281). Within are the dark-coloured spores, 

 which on opening a single sporangium carefully in a drop of glycerine 

 may be counted to the number of 48. Normally the sporangia open 

 in dry air, and the dry and dusty spores are forcibly thrown out. 



The origin of a sporangium is by outgrowth of a single superficial cell of the 

 receptacle, which undergoes successive segmentations as illustrated in Fig. 279, 

 1-3. A tetrahedral internal cell is thus completely segmented off from a single 

 layer of superficial cells constituting the wall. The former undergoes further 



