544 



THE INDIVIUDAL ORGANISM 



walled. By the time the zone of elongation has progressed beyond any 

 given group of epidermal cells, they have produced delicate tubular out- 

 growths, the root hairs, from a fraction of a millimeter to a centimeter in 

 length, closed at the free ends. Thus the surface of the zone of maturation 

 that follows the region of elongation becomes clothed with a dense growth 

 of root hairs which penetrate the soil crevices and absorb water and 

 dissolved substances through their thin walls. The region from the root 



stele 



cortex 



region of 

 maturation 



and 

 root hairs 



epidermis 



pericycle 

 endodermis 



root hairs 



region of 

 elongation 



epidermis 



cortical 

 parenchyma 



pericycle 



xyte 



growmg pent j^UM^MHUUHa^Mdodro 



root cap 



Fig. 10.2. The structure of a dicot root. A, longitudinal section of root tip. B, part of a 

 cross section in the region of maturation. C, part of last, more highly magnified, showing 

 stele and surrounding tissues. (Modified from Turtox chart, courtesy General Biological Supply 

 House, Inc.) 



cap to the beginning of the root-hair zone is also absorptive, but its 

 surface is so small compared to that of the mass of root hairs that the 

 latter are many hundredfold as effective. As the root grows forward, new 

 root hairs are continually produced at the distal 1 border of the root-hair 

 zone, and those at the proximal 1 border die. Behind the region of root 

 hairs the surface of the root becomes nonabsorptive because of the deposi- 



1 These terms are useful in describing position and direction in relation to pro- 

 jecting structures, such as an arm or a plant root. The tip is the distal end, the attached 

 base, the proximal end; but with reference to any point along the structure, distal 

 means the direction away from the base; proximal, the direction away from the free 

 end. 



