THE ROOT. 73 



The number of vascular strands constituting the stele is 

 various, being as few as four or as many as forty. The 

 ordinary number, however, is from eight to twenty. (See 

 figs. 86, 89.) 



82. 3. The cortex generally consists of large thin-walled 

 cells which have become partially separated from each other, 

 leaving larger or smaller intercellular spaces (figs. 86, 89). 

 Its innermost layer, bordering the stele, is usually quite 

 different from the rest, and is recognizable by its wavy, radial 

 walls, which are suberized (^[ 9). This layer is called the 

 endodermis (figs. 86, 88, 89). 



83. 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 parts where they are useful. The 

 secondary roots themselves may be either permanent or tran- 

 sient. 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. 



84. 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 secondary 

 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. 



85. 1. External secondary changes. — In some cases the 

 older roots differ from the younger in scarcely more than the 



