124 PLANTS, THE FOOD OF ANIMALS 



leaves. In nature, sporulation of Pteridium usually occurs in 

 August, and in allied forms sometime during the summer 

 months. The margins of the mature leaves, 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 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 Pteridium. 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 (Gametophyte) . 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 



FIG. 50. Germination of the spore and formation of the prothallium. A, 

 Young plant leaving the spore case; B, similar stage after one cell division has 

 occurred (p, protenema; s, spore case; r, root). Later stages in formation of the 

 young prothallium are shown on the left, and below a fully developed prothallium 

 with archegonia and antheridia. In the notch above is a figure (life size) of the 

 same prothallium. (From Sedgwick and Wilson after Suminski.) 



