THE ROOT. 71 



root, therefore, is without root-hairs because' of their death. 

 The youngest part of the root is likewise free from them, 

 because they have not yet been produced. As the root grows 

 in length, new root-hairs are continually being produced and 

 the older ones are dying at an equal rale, so that a zone 

 of hairs is found only upon the younger parts of the roots. 



80. (b) The root-cap, serving to protect the tenderer por- 

 tion of the root behind, is itself constantly exposed to injury. 

 The outer and older cells of the root-cap are, therefore, either 

 torn away through mechanical contact, having become gradu- 

 ally loosened from each other with age; or, losing their 

 active contents, they degenerate and break down into a 

 slightly mucilaginous material which facilitates the passage of 

 the root through the substratum. This degeneration or the 

 mechanical wear is repaired constantly by the formation of 

 new cells in the growing point. The thickness of the root- 

 cap, therefore, is maintained throughout its existence without 

 considerable change. It rarely becomes more than a few cell- 

 layers thick. Since its tissue is produced only by the division 

 of the apical cell or cells, it is organically connected with the 

 root only at the very tip; but it usually extends backward over 

 the root, by reason of its growth, for a considerable distance. 

 If the finger be supposed to represent the root, a short finger- 

 stall, if it were attached to the tip of the finger, might be 

 fairly taken to represent the position of the root-cap. Only 

 in rare cases is the root -cap entirely wanting. 



81. 2. The stele. Occupying the center of the root, and 

 surrounded on all sides by the cortex, is an aggregate of 

 tissues called the central cylinder, or stele (figs. 84, 86, 89). 

 The outermost layer of its cells is the pericycle (figs. 86 88, 

 89). Within this are found strands of elongated cells or 

 cell-fusions,* called vascular bundles, or strands. These 



* These are continuous chambers formed by the breaking down of the 

 partition-walls between the abutting ends of cells. They are usually 

 devoid of living contents. 



