THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



635 



Equisetiim the spores are all alike, although recent investigations have shown 

 that there is a slight variation in the size of the spores, suggesting that possibly 

 we have here the beginning of a heterosporous condition. Equisetiim, there- 

 fore, may be said to occupy an intermediate position between the Fern with 

 a purely homosporous sporangium giving rise to only one type of free-living 

 prothalius, and Selaginella with its typical heterosporous condition and 

 unisexual prothalli. 



The life-history oi Equisetiim can therefore be illustrated by Fig. 643. 



STEM APEX 



LEAF 



ROOT 



EQUISETUM 



STEM APEX 



FOOT 



SUSPENSOR 

 RHIZOPHORE 



FOOT 



SELAGINELLA 



FOOT 



STEM APEX 



LEAF 



MARATTIA 



Fig. 646. — Comparative embryology of Equisetiim, Selaginella and Marattia. 

 F = foot; L=leaf; k = root; S= stem apex. 



