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 
Fic. 6. 
Later stages of segmentation of the sporangium of Nephrodium 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. 5.11, 12) to form a 
second layer of transitory nutritive cells, called the tapetum, subsequently 
doubled by tangential fission (Fig. 6.1). The tetrahedral cell which 
