496 



BOTANY OF THE LIVING PLANT 



A vertical section through the sorus of Dryopteris shows an 

 enlarged receptacle, traversed by the vascular strand. The indusium 

 rising from it overarches the numerous sporangia which are attached 

 basally by long thin stalks (Fig. 387). The head of each sporangium is 



Fig. 388. 

 Successive young stages in the segmentation of the sporangium of Dryopteris 



Filix-mas. (After Kny.) 



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 place (Figs. 387, 389, 4a). Within are the dark-coloured 

 spores, which on opening a ripe 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. 



