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 1 , who described the 



.-v.b. 



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. Callitriche stagnalis, Scop. Central cylinder of stem. A, land form, 

 (x 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 



1 Sanio, C. (1865). 



