CH. I] 



THE SPORANGIUM 



overarches the numerous sporangia which are attached to it by long thin 

 stalks (Fig. 14). The head of each sporangium is shaped like a biconvex 

 lens; its margin is almost completely surrounded by a series of indurated 

 cells, which form the mechanically effective annuliis. This stops short on 

 one side, where several thin-walled cells define the stomium, or point where 

 dehiscence will take place (Figs. 14, 16,4^ 17). 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. 



Later stages of development of the sporangium of 

 Dryopteris Filix-mas. (After Kny.) 



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

 which undergoes successive segmentations as illustrated in Fig. 15, 1-3. A tetrahedral 

 internal cell is thus completely segmented off from a single layer of superficial cells con- 

 stituting the wall. The former undergoes further segmentation to form a second layer of 

 transitory nutritive cells called the tapetum (Fig. 15, 6-12), subsequently doubled by tan- 

 gential fission (Fig. 16, i). The tetrahedral cell which still remains in the centre, having 

 grown meanwhile, undergoes successive divisions till 12 spore-mother-cells are formed 

 (Fig. 16, 2-7). These become spherical, and are suspended in a fluid which, together with 



