Roots and Their Environment 



187 



are totally submerged. The roots of hydrophytes, like the 

 leaves and stems, are remarkable for the presence of internal 

 air cavities (page 156). 



WTien the roots of .^, 

 land plants (mesophytes) '^ 

 extend into well-aerated | 

 water, they develop in- (' 

 numerable branches, dif- ' 

 fering in this respect - • 

 very markedly from the 

 roots of hydrophytes. 

 On account of this fact, 

 roots of trees, especially 

 those of willow and Cot- 

 tonwood, that enter V 

 drain pipes and tiles 

 often develop masses of ^ 

 fine branches that ob- — 

 struct the flow of the -- 

 water even when the 



entermg root is not Y\g. 107. Holdfast roots of trumpet creeper, de- 

 thicker than the lead in vcloped from the nodes. These roots are perennial 

 •1 n^i -LI r and may lengthen and branch for several years. 



a pencil. Ihe banks of 



streams are often protected from erosion by the mat of roots 

 developed along the water's edge. This is why willows are 

 planted on levees. 



In moderately dry regions the roots of xerophytes may 

 penetrate to considerable depths, but in deserts many of the 

 largest plants have only small root systems, spread in the 

 upper layers of the soil (page 184). 



Climbing plants, like the Virginia creeper, poison i\^, 



