XIV 



ISOETACEJE 



555 



in number in tlie fully-developed leaf, and the dia])liraj[^'ms 

 across them more rei^ular and coni])lele. Instead of ljein<^" 

 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 ovcr-la])])ing leaf bases gi\-e the whole stem 

 much the appearance of the scal\- bulb oi many Monocotyledons. 



Fig. 321. — Tsoetcs lacustn's. 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 wdiich is (3) the main portion of the ligule, 

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

 composed of dead cells. 



