192 Mineral Nutrition of Plants 



ferentiation of the endodermis, especially the development of the suber- 

 ized Casparian strip. A transverse section of a typical root through the 

 transition region of elongation and maturation is reproduced from 

 Priestley (6j) as Figure 1. Certain features may be particularly ob- 

 served, namely, the relative areas of cortex and stele, the lack of inter- 

 cellular spaces between cells and vessels within the stele and their prom- 

 inence in the cortex, the differentiation of the endodermis and passage 

 cells therein, and the nature of Casparian strip development and the 

 development of root hairs which increase in number in the region of 

 maturation. These anatomical features no doubt play a part in deter- 

 mining the environmental conditions and the rates of processes within 

 the various tissues of roots. 



Longitudinally, root structure is distinguished by three portions which 

 grade from one into the other in passing from the tip to the root-stem 

 junction; the extent of each depends upon the relative rates of pro- 

 cesses within and differentiation of the component cells and tissues. 

 These three, respectively, have been termed regions of meristematic 

 development, elongation, and maturation. The longitudinal gradation 

 of effectiveness for penetration and movement of materials across tis- 

 tues is reviewed by Wiersum (92). Movement of solutes across the 

 meristematic region and along its walls is possibly restricted by the 

 close packing of cells, the walls of which are reported to be impreg- 

 nated with protein. Most effective independent flux is considered to 

 occur into the region of elongation. Attended by progressive differenti- 

 ation in tissues farther from the tip, the rate of transverse independent 

 migration of solutes to the stele is considered to be reduced again. How- 

 ever, in this region of maturation, rates of solute influx to the stele may 

 be modified and increased where root laterals are initiated from within 

 the pericycle, thus effectively "piercing" a possible endodermal barrier. 

 The same general pattern has been pictured by Popesco (60). His 

 studies have laid more particular stress on the development of root hairs 

 in the region of maturation. Many investigators have questioned the 

 importance of root hairs in absorption since their pronounced effective- 

 ness is limited to conditions in soils where they may effectively increase 

 the external surface to media of low moisture and/or deficient in- 

 organic solute content. 



