3j8 



BOTANY OF THE LIVING PLANT 



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

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

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

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

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



Fig. zyg. 



Succcssi\e young stages in the segmentation of tlie sporangium of Kcphfodiiim 



Filix-mas. (After Kny.) 



place (Figs. 278, 280, J^a, 2S1). \A'ithin arc 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 sporangiuni is by outgrowth of a single superficial cell of the 

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

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

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



