176 AQUATIC STEMS [ch. 



sation. The vascular tissue is concentrated into a definite cylin- 

 der, with external phloem and internal xylem, enclosing what 

 seems at first sight to be a pith. But Sanio^, who described the 



v.b. 



..s. 



Fig. 113. Ranunculus tnchophyllus, Chaix. T.S. young stem to show the numerous 

 air spaces, s, in the ground tissue, v.b. = vascular bundle; h = hair, (x 47.) 



[A. A.] 



Fig. 114. Callitricke stagnalis, Scop. Central cylinder of stem. A, land form. 

 [^ 475-) B> water form, (x 290.) [Schenck, H. (1886).] 



anatomy of the stem, demonstrated that the central region, 

 which, if the mature structure alone were examined, would 



iSanio, C. (1865). 



