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 phloém, 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- 
