CHAPTER I. GENERAL PHYSIOLOGY. 



681 



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- 

 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. 159). It appears that the development of root-hairs is de- 

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



FIG. 468. A Root-hairs (7i) on the primary root (10) of a seedling, grown in water of 

 Buckwheat (Polygonum Fagop>,rum); Tic 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. 



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

 root-hairs are usually not developed by aquatic plants, the roots of 

 which, at least, are habitually immersed in water; nor by plants 

 in which the transpiring surface is relatively small in proportion 

 to the root-system (e.g. small-leaved Conifers; saprophytes, such 



