INFLUENCE OF ENVIRONMENT ON PLANTS 3387 
or it may be altogether floating. The stems are generally 
long and slender, and easily swayed to and fro in the water. 
‘Some have, however, very short stems which give rise to 
numerous elongated ribbon-like leaves. These flexible 
stems depend for their support upon the nature of the 
medium in which they live, and though they possess a 
—— 
SIE 
Fig. 139.—SEction or Stem or Potamogeton, snowina AIR PassacEs 
IN THE CoRTEX. 
certain rigidity, this is not associated with any great de- 
velopment of woody tissue. Generally the latter is reduced 
to a minimum; the fibro-vascular bundles are usually few 
and contain few lignified elements. Their substance is 
largely parenchymatous, and the cells have thin walls. 
The intercellular space system is often very complex, large 
lacune filled with air occupying considerable space in the 
distribution of the tissues (fig. 189). Their rigidity is 
22 
