THE ROOT. 65 



77. Duration, — Even when the primary root persists 

 throughout the entire life of the plant secondary roots often 

 appear. When the primary root perishes, its functions must 

 be performed wholly by secondary roots, which are developed 

 in succession upon those parte where they are useful. The 

 secondary roots themselves may be either permanent or 

 transient. In creeping plants particularly, whether growing 

 on land or in water, the functions of the root are likely to be 

 handed on to successively younger roots, the old ones perish- 

 ing and dropping off. If the roots endure for a considerable 

 time, they may retain their primitive structure and form, or 

 they may undergo secondary changes which unfit them for 

 absorbing organs, and adapt them to subserve various special 

 functions. 



78. Secondary changes. — Shortly after any portion of 

 the root has ceased to increase in length, and, therefore, 

 within the first season, it ordinarily undergoes minor second- 

 ary changes which may or may not be followed by more 

 profound alterations. These changes affect its primary 

 structure in various ways and to various degrees according to 

 the parts concerned. 



In some cases the older roots differ from the younger in 

 scarcely more than the loss of the external layer of cells, from 

 which the root-hairs arose. The sloughing off of this layer 

 carries with it the hairs themselves and exposes the next inner 

 layer of cells, which had before become slightly altered so as 

 to be rather impervious to water. Upon their exposure, this 

 alteration proceeds further, so that they become almost or 

 quite incapable of absorbing the soil-water to which they may 

 be exposed. It follows from this that it is only the younger 

 part of the root, that is, the portion which has not undergone 

 secondary changes, which is capable of absorbing water. In 

 many roots this is the only change which occurs. In a 

 greater number the root is also strengthened. 



