168 



PART III. PHYSIOLOGY. 



[41 



In a typical land-plant the development of the root-system is 

 such as to ensure an adequate supply of food from the soil, and a 

 supply of water sufficient to maintain the general turgidity of 

 the plant in spite of continued loss of water by transpiration. 



The root of such a plant is adapted for the performance of its 

 functions both in its structure and in its properties. The most 

 striking structural adaptation is that the walls of the superficial 

 cells of the younger parts are not cuticularised, but remain per- 



Fi. 123. A. Root-hairs (7i) on the primary root (ic) of a seedling grown in water of 

 Buckwheat (Polygonum Fagopyrum) ; he hypocotyl ; c cotyledons. B (after Sachs) Ends of 

 root-hairs showing their intimate connexion with particles of soil which adhere to the 

 mucilaginous external layer of the cell-walls. 



vious to water. Generally speaking, the absorbent area of the 

 root is increased by branching ; and, in many cases, also by the 

 growing-out of the superficial cells of this region into root-hairs 

 (see p. 46). It appears that the development of root-hairs is de- 

 termined by the difficulty of obtaining water, on the one hand, 

 and by the relative activity of transpiration on the other : thus 



