138 THE EVOLUTION OF PLANTS 



The Club-mosses, unlike the Ferns, have 

 small and simple, but numerous leaves, borne, as 

 a rule, on long and conspicuous forked stems, 

 the habit being like that of a Moss on a large 

 scale. The chief exception is Isoetes, a genus 

 mostly aquatic, in which the stem is much stunted, 

 and the leaves, though simple, are comparatively 

 long. The spore-sacs are invariably borne singly 

 on each leaf, either on its upper surface, or in the 

 angle between leaf and stem. Sometimes the 

 leaves associated with the spore-sacs are ordinary 

 leaves; more often they are special sporophylls, 

 grouped together to form a definite cone. The 

 class includes many plants with one kind of spore, 

 and still more numerous species in which micro- 

 spores and megaspores are differentiated. Ly co- 

 podium and Phylloglossum represent the former, 

 Selaginella and IsoStes the latter condition. 



The Horsetails are widely different from both 

 the other classes. The leaves are extremely 

 reduced, appearing as mere teeth on the sheath 

 formed by the united leaf-bases. The stems 

 are well developed and jointed, the leaves and 

 branches being in whorls. The cones are very 

 sharply marked off; the spore-sacs are borne on 

 the under side of shield-like scales; the spores are 

 all of one kind. 



We will begin with the Ferns, because they 

 appear to come nearest, as we have seen in the 



