80 



FOUNDATIONS OF BIOLOGY 



of this type form the vital point of contact between the root 

 and its environment. The root hairs exhibit the selective 

 power of protoplasm to a remarkable degree. For example, 

 Red Clover plants and Barley plants when burned yield about 

 the same proportion of mineral matter as ash. But the 



Barley ash contains nearly 

 twenty times as much 

 silica as the Clover, while 

 the latter contains nearly 

 six times as much lime as 

 the Barley. (Fig. 44.) 



In the zone in which 

 root hairs are present the 

 central cylinder of the 

 root shows still more cell- 

 ular specialization. The 

 young vascular bundles 

 are differentiated into the 

 phloem tissue, character- 

 ized by its small angular 

 ducts; while the develop- 

 ing xylem, with its large 



FIG. 44. Root hair (very highly magnified) ducts, has obliterated the 

 showing its relation to adjoining cells of the root ... , 



and to particles of the soil, a, vacuole filled primitive ground tlSSUC, Or 

 with cell sap; b, cytoplasm (dotted); c soil ith Between the Xylem 



particles; d, nucleus within the cytoplasm lining * 



the ceil wail. and the phloem appears 



the developing cambium, but this begins its characteristic 

 growth contribution somewhat above the hair zone. Indeed, 

 as we pass upward from the region where the root hairs are 

 developed, the cellular structure becomes more and more 

 similar to that of the stem; the older woody roots of trees 

 and shrubs being, from the standpoint of both structure and 

 function, stems. 



