178 AQUATIC STEMS [CH. 



'pith' are actually xylem, and not merely altered pith cells, is 

 indicated by their possession of typical tracheal thickenings, 

 and also by their occasionally identifying themselves, as just 

 mentioned, with the xylem ring. 



lA' X)-n O 



O L+L' 

 L 



FIG. 115. Hippuris vulgaris, L. Series of transverse sections of stele of a stem near 

 apex to show relation of cauline and leaf trace xylem ; the dotted line in each case 

 represents the periphery of the stele. ( x 280 circa.) A , appearance of first cauline 

 element when stele is 0-08 mm. in diameter. B, level at which first lignified leaf 

 trace begins to pass in towards the stele, which contains 21 cauline xylem elements, 

 but no leaf traces. C, the level at which eight lignified leaf traces (L) have taken 

 up a position at the periphery of the stele, in which 21 cauline elements can still 

 be counted. D, a lower level at which traces (L') from a second node have entered. 

 Fusion of traces from the two nodes or of cauline elements with either is indicated 

 by (L + L 1 ), (C + L), etc. [A. A.] 



Myriophyllum (Figs. 1 16 and 117) closely resembles Hippuris 

 in vascular anatomy and has the same cauline tracheal elements 

 in the pith, but the xylem is more reduced 1 . 

 1 Vochting, H. (1872). 



