22 



THE LIFE-HISTORY OF A FERN 



how the indusium, rising from the receptacle, overarches the sporangia, 

 which are also attached to it by long thin stalks. The head of each 

 sporangium is shaped like a biconvex lens : its margin is almost com- 

 pletely surrounded 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 shall take place. 

 Within are the dark-coloured spores, which, on opening a single sporangium 



FIG. 6. 

 Later stages of segmentation of the sporangium of Nephrodiuvi Filix-mas. (After Kny.) 



carefully in' a drop of glycerine, may be counted to the number of 

 approximately 48. 



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

 cell of the receptacle, which undergoes successive segmentations as 

 illustrated in Figs. 5 : 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 segmentation (Fig. 5. n, 12) to form a 

 second layer of transitory nutritive cells, called the tapetum, subsequently 

 doubled by tangential fission (Fig. 6. i). The tetrahedral cell which 



