172 VASCULAR SYSTEM OF AQUATICS 



epidermal cells have thin outer walls with a very thin cuticle, 

 and there are no stomata. 



The structure of the leaf, as seen in cross-section, is essentially 

 similar to that of the stem. Its centric character may perhaps 

 be related to the absence of illumination from any particular 

 direction, owing to the diffuse character of the light and the 

 constant movement of the leaves by the water. 1 



Those aquatics, whose lower parts only are submerged, 

 approach more nearly to land-plants in their internal structure. 

 In theMare's-tail (Hippuris, Fig. 90) , for instance, the xylem (Xy.), 

 which is necessary to supply water to the aerial parts, though 

 not extensive, forms an obvious zone situated immediately 

 within the phloem (p.) . Moreover, there is a large central pith (pi.) 

 whose presence can be related to the growth of the shoots above the 

 water and the consequent bending strains to which these are 

 exposed. Hence a more peripheral disposition of the mechanical 

 elements than occurs in submerged plants is rendered necessary. 



Even the more extreme aquatics, however, often preserve 

 indications of their terrestrial ancestry in their vascular system. 

 Thus spiral vessels commonly persist at the nodes (e.g. Pota- 

 mogetori), and transient spiral vessels occur in the young 

 internodes. In the different species of Pondweed can be 

 found various stages of the concentration and reduction of 

 the vascular system. These features are least pronounced in 

 the Floating Pondweed (Potamogeton natans), which develops a 

 relatively large inflorescence rising some two or three inches out of 

 the water, and further possesses floating leaves. The large central 

 cylinder (Fig. 91) encloses eight to ten fairly well-defined vascular 

 bundles, each of which is separated from its neighbours by two 

 to three layers of thin-walled parenchyma, and consists of a 

 large xylem-canal (C.) with accompanying phloem (P.). The 

 likewise broad-leaved, but completely submerged, P. lucens pre- 

 sents a more marked concentration of the bundles, whose indi- 

 viduality is consequently less pronounced. Lastly, the narrow- 

 leaved P. pectinatus, which is also totally submerged, exhibits 

 but a single xylem-canal surrounded by phloem, as in Cerato- 



1 A similar reduced structure is encountered in the Fennel-leaved 

 Pondweed (Potamogeton pectinatus) and in completely submerged marine 

 aquatics (e.g. the Grasswrack, Zoster a). 



