FILICALES 



339 



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

 tapeium (Fig. 279, 6-12), subsequentlydoubled by tangential fission (Fig 280,1). 

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

 while, undergoes successive divisions till twelve spore-mother-cells are formed 

 (Fig. 280, 2-7). These become spherical, and are suspended in a fluid which, 

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



FIG. 280. 



Later stages of development of the sporangium of Nephrodium FiHx-mas. 

 (After Kny.) 



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 

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

 are readily scattered. Since each of the 12 spore-mother-cells forms four 

 spores, their number is -4&in each sporangium. Each mature spore consists 

 of a nucleated protoplast, bounded by a colourless inner wall, and a brown 

 epispore bearing irregular projecting folds. 



