AQUATICS, STRUCTURE 



171 



rounding water, so that protection is alone required against the 

 longitudinal tension due to the force of the current. A small 

 intercellular space (c), surrounded by three or four layers of 

 thin-walled parenchyma-cells, occupies the centre of the strand. 

 This space is actually a longitudinal canal (the xylem-canal) 

 formed by the breaking down of elements of the procambial 

 strand which, in less extreme aquatics [e.g. Water Milfoil, Myrio- 

 phyllum) , give rise to xylem. Beyond the enveloping parenchyma 

 lies the phloem, which can be recognised by its large, empty- 

 looking sieve-tubes (Fig. 89, s.t.). The absence of xylem, whilst 



Fig. 89. — Habit, and transverse section of the stem, of the Hornwort {Cera- 

 tophyllum demersum). c, xylem-canal ; en., endodermis ; 5., inter- 

 cellular space of cortex ; s.t., sieve-tube. 



the phloem is well represented, can be related to the fact that 

 the aquatic absorbs water over its whole surface, whilst con- 

 duction of elaborated food-materials remains as necessary as in 

 a terrestrial plant. 



The vascular strand is sharply bounded towards the cortex 

 by a well-defined endodermis (Figs. 8g and 90, en.), showing 

 suberised thickenings on the radial walls, and immediatelj? wthin 

 is a pericycle, just as in many young roots. The wide cortex 

 beyond consists of thin-walled parenchyma in which a ring of 

 large intercellular spaces (s.) surrounds the central strand. The 



