XIV 



ISOETACEM 



555 



in number in the fully-developed leaf, and the diaphragms 

 across them more regular and complete. Instead of being 

 throughout but one cell thick, as in the first leaves, they are 

 thicker at the edges, so that in section they appear biconcave. 

 In the older leaves the broad sheath at the base is much better 

 developed, and the over-lapping leaf bases give the whole stem 

 much the appearance of the scaly bulb of many Monocotyledons. 



FIG. 321. Isoetes lacusti 



Section of root-apex, showing dichotomy, X about 190 

 (after Bruchmann). 



In all the terrestrial species, and those that are but partially im- 

 mersed, the leaves are provided with numerous stomata of the 

 ordinary form ; but in some of the submersed species these are 

 partially or entirely wanting. The development of the ligule 

 also varies, being very much greater in the terrestrial species, 

 where it may possibly be an organ of protection for the younger 

 leaves. 



The ligule in its fully developed condition (Smith (i)) 

 shows four portions: I, a sheath of glandular appearing cells 

 at its base ; 2, the "glossopodium," consisting of a band of large 

 empty cells, above which is (3) the main portion of the ligule, 

 composed of small cells containing protoplasm; 4, the apex, 

 composed of dead cells. 



