FILICALES 



497 



The origin of a sporangium is by outgrowth of a single supcrliu.il < ell of the 

 receptacle, which undergoes successive segmentations as illustrated in Fig. 388. 

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

 layer of superficial cells constituting the wall. The Conner undergoes fun 

 segmentation to form a second layer of transitory nutritive cells called the 

 tapetum (Fig. 388, 6-12), subsequently doubled by tangential fission (Fig. 389,1). 



Fig. 389. 



Later stages of development of the sporangium of Dryopteris b'ilix-mas. 



(After Kny.) 



The tetrahedral cell which still remains in the centre, having grown mean- 

 while, undergoes successive divisions till twelve sporc-mothcr-cclh are formed 

 (Fig. 389, 2-7). These become spherical, and are suspended in a liquid which, 

 together with the now disorganised tapetum, fills the enlarged cavity of the 

 sporangium. Each' spore-mother-cell then divides twice to form a Spore- 

 tetrad: in this process, just as in the formation of pollen-grains and other 

 spores, the number of chromosomes is reduced to a half. Finally the resulting 

 cells separate on ripening as individual spores, each covered by a protecting 

 wall, rugged and dark brown at maturity. Owing to the absorption of the 

 liquid contents of the sporangium the separate spores are dry and dusty, and 



