THE ROOT. 



63 



duced and the older ones are dying at an equal rate, so that 

 a zone of hairs is found only upon the younger parts of the 

 roots. 



74. {b) The root-cap. — If the finger be supposed to rep- 

 resent 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. Serving to protect the tenderer 

 portion of the root behind, the root-cap is 

 itself constantly exposed to injury. The 

 outer and older parts of the root- cap are, 

 therefore, either worn away through me- 

 chanical contact ; or, dying, they degener- 

 ate and break down into a slightly muci- 

 laginous material which facilitates the 

 passage of the root through the substratum. 

 This degeneration or the mechanical wear 

 is constantly repaired within at the grow- 

 ing point. The thickness of the root- 

 cap, therefore, is maintained throughout 

 its existence without considerable change. 



75. 2. The stele. — Occupying the cen- 

 ter of the root, and surrounded on all sides 

 by the cortex, is an aggregate of tissues 

 called the central cylinder, ox stele (figs. 51, 

 53). The most noticeable part of this are 

 the groups of elongated cells or cell- 

 fusions,* called vascular bundles, or vas- 

 cular strands. These strands are of two 

 kinds, wood strands, specially for the con- 



FlG. 52. — A nearly ma- 

 ture root-hair, showing 

 structure and relation 

 to superiicial cell of 

 root ; grown in water 

 and therefore not dis- 

 torted as in fig. 51. 

 », nucleus embedded 

 in protoplasm; vacuole 

 single and verjj large. 

 Highly magnified. 

 —After Frank. 



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

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

 void of living contents. 



