REPRODUCTION OF THE FERN 121 



from spores which develop on the under surfaces of the leaves. 

 In nature sporulation of Pteris usually occurs in August and 

 in allied forms sometime during the summer months. The 

 margins of the mature leaves of Pteris when ready for spore 

 formation turn under and form elongated pockets which extend 

 throughout the length of the pinnules. This inturned shelf of 

 tissue is termed the false indusium, while another shelf of tissue 

 derived from the epidermis of the under surface and extending 

 out to the false indusium, is called the indusium, the spore- 

 bearing organs being formed in the chamber enclosed by the 

 true and false indusia and the enclosed under surface of the 

 pinnule. 



In other types of fern the spore chambers are somewhat 

 differently constructed. In the maiden-hair for example, the 

 entire edge of the pinna is not turned in, but three or more spots 

 on the edge become localized spore-forming centers each cov- 

 ered by an indusium. In the Boston fern a row of similar 

 spots on each side of the median line on the under surface 

 are spore-forming centers, each spot, termed a sorus, is covered 

 by an indusium. 



The spores develop in peculiarly shaped spore-cases called 

 sporangia, many of which are formed in a sorus and multitudes 

 in the spore chambers of Pteris. Each sporangium begins by 

 the division of an epidermal cell (Fig. 49, a-h) until a capsule is 

 formed with a ridge (annulus) of specially hardened cells. 

 Within the capsule a single germ cell, the archesporium, di- 

 vides six consecutive times forming 64 spores, each spore being 

 enclosed in a firm covering (shell or epispore, Fig. 49, 1) . When 

 ripe the sporangium bursts open by contraction of the cells 

 of the annulus and the spores are scattered from the leaves to 

 the ground. 



The Sexual Generation. After some months on the ground the 

 spores absorb moisture, the epispore bursts open and the 

 spore cell or endospore begins to swell and to divide forming 

 root-like hairs (rhizoids) and embryonic plant termed the 

 protonema (Fig. 50). The end cells of the protonema develop 

 chlorophyll, divide and ultimately form a flattened plate of 



