1118 



In both of the supposed species, the stem consists of a hollow tube, 

 the central cavit}^ of which many times exceeds in diameter the solid 

 portion; in which respect they differ conspicuously from all the other 

 species. The siliceous cuticle in both is identical : in the exposed 

 portions of each it is equally penetrated by stomata, which are neces- 

 sarily absent from its submerged parts. The solid portion of the stem 

 in both consists of regular, well-defined, hexagonal cells. 



A — Transverse section of the stem of E. liraosum, magnified. 1. Hollow tubes ; 2. Triangular system of 

 delicate hexagonal cells. 



B. — Transverse section of the stem of E. fluviatile, magnified as in A. 1. Inner row of circular tubes; 2. 

 ( (uter row of plano-convex tubes. 



In E. fluviatile it is about twice as thick as in E. limosum, and is 

 penetrated by two series of tubes, equalling in number the external 

 striaj. The inner row of these, which apparently contain the delicate 

 spiral vessels and annular ducts, are circular in shape, and placed 

 close to the central cavity. Behind these, but at a considerable space 

 from the epidermis^ alternate the second row of tubes, which are 

 plano-convex or elliptical in shape ; the major axis being double the 

 length of the minor. 



The solid portion of the stem of E. limosnm is only about half as 

 thick as that of E. fluviatile, and is consequently much less succu- 

 lent. It is penetrated by only a single row of tubes, which, in their 

 organizatin and position, resemble the inner row in E. fluviatile. 

 The epidermis opposite these is depressed, and bears a system of 



