122 PLANT LIFE 



the water plants consists in the degenerate 

 character of the wood. This poverty in 

 water-conducting tissue is, however, chiefly 

 to be seen in the parts which have elongated. 

 The nodes, whence the leaves arise, often 

 exhibit quite a considerable amount of vessels 

 and tracheids. These parts of the stem, 

 which do not elongate, are in sharp contrast 

 to the internodes, in which practically all the 

 growth in length of a stem takes place. In 

 the internodes the wood is often merely 

 rudimentary, and it may be absent alto- 

 gether. On the other hand, the phloem, in 

 which the organic substances- mainly travel, 

 is usually as well developed as in a land plant, 

 and sometimes even better, relatively speaking. 

 As regards the mechanical tissues, aquatics 

 are specially interesting. They are ^almost 

 of the same specific gravity as the water, and 

 when the air-spaces are taken into account 

 they are usually much lighter. Consequently, 

 arrangements for providing the mechanical 

 condition of support are unnecessary and 

 would be wasteful. The only serious me- 

 chanical requirements are those adapted to 

 prevent the plants growing in swift torrents 

 being torn asunder by the force of the current. 

 We find that, on the whole, the mechanical 

 tissue, when present, and also the vascular 

 strands, tend to occupy an axile position. 

 This is especially advantageous for the latter, 

 as the bending and waving movement of the 

 flexible stems will naturally cause the mini- 



