INFLUENCE OF ENVIRONMENT ON PLANTS 387 



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 



FIG. 139. SECTION OF STEM OF Potamogeton, SHOWING AIR PASSAGES 

 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 

 lacunae filled with air occupying considerable space in the 

 distribution of the tissues (fig. 139). Their rigidity is 



22 



