1 86 General Botany 



erosion. In the same way the crowns of clover and plantain roots 

 that have been lifted up by frosts may be drawn into the soil, and 

 small bulbs and tubers, many of which are formed at higher 

 levels than the parent bulbs, may be pulled deeper into the soil 

 by root contraction. 



Root duration. Roots are annual, biennial, or perennial. 

 Perennial plants may have either annual or perennial roots, just as 

 they may have either annual or perennial aerial stems. Plants 

 with bulbs, tubers, or corms grow a new set of roots each year. 

 Plants with rootstocks, like the May apple and Solomon's seal? 

 generally have roots that last for several years. Shrubs and trees 

 also have perennial roots. We must be sure to understand, how- 

 ever, that even in perennial roots the work of absorption is for the 

 most part done by the youngest portions of the new roots which 

 are added each year. Most biennials, like the common evening 

 primrose and wild carrot, have fleshy roots in which food accumu- 

 lated during the first year. This food is used in the rapid develop- 

 ment of the plant during the second season. 



Roots of hydrophytes. Most of the root characteristics thus 

 far described are those of the roots of mesophytes. In hydro- 

 phytes the roots are notably smaller and less branched than in 

 mesophytes. They absorb water and mineral substances from 

 the soil, even when the plants are totally submerged. The roots 

 of hydrophytes, like the leaves and stems, are remarkable for the 

 presence of internal air cavities. 



When the roots of land plants (mesophytes) extend into well- 

 aerated water, they develop innumerable branches, differing in 

 this respect very markedly from the roots of hydrophytes. On 

 account of this fact, roots of trees, especially those of willow and 

 Cottonwood, that enter drainpipes and tiles often develop masses 

 of fine branches that obstruct the flow of the water even when 

 the entering root is not thicker than the lead in a pencil. The 

 banks of streams are often protected from erosion by the mat of 

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



