388 A TEXTBOOK OF GENERAL BOTANY 

Fic. 480. Sporangia of a fern (Pteris longifolia) 
Left, entire sporangium ; center, sporangium opened by the bending back of 
the annulus (the row of thick-walled cells) ; right, an annulus has returned to its 
original position, which movement resulted in the scattering of the spores. (x 85) 

Fig. 431. Germination of fern 
spore and early stages in growth 
of prothalli 
phloém has disappeared and in 
which there has been a massive 
development of secondary xylem. 
The sporangia of ferns are usu- 
ally borne on the lower surfaces 
of the leaves (Fig. 424) and are 
aggregated in definite areas, the 
sort. In each sporangium there are 
many spores (Fig. 430). When a 
spore germinates, it produces a 
small green thallus, the gameto- 
phyte (Figs. 431, 482). 
Gametophyte. The gameto- 
phyte of the Ptertdophyta is 
known as the prothallus (Fig. 
432). It is typically a delicate, 
thin, expanded thallus which is 
attached to the ground by means of numerous rhizoids. The 
latter are slender filaments which absorb water and mineral 
matter just as do the root hairs of flowering plants. The typical 
prothallus is an independent plant which manufactures its food 
