ADJUSTMENT TO WATER 43 



the zone of root-hairs the outer wall of the epidermal cells is 

 rendered more or less impervious, i.e., cutinized, in order to pre- 

 vent the cells from drying. A similar fate apparently overtakes 

 the root-hair cells after the hair has shriveled and disappeared. 



56. Effect of water content upon root-hairs and roots. The 

 formation of root-hairs is closely connected with the amount of 

 water present. Water plants do not form root-hairs at all, or 

 only to a small degree. The same species, indeed the same root, 

 will form abundant root-hairs in moderately dry soil or in moist 

 air, while in the water few or no hairs are produced. The rea- 

 son for this seems evident, when it is borne in mind that root- 

 hairs are primarily for the purpose of increasing the absorbing 

 surface. In streams, ponds, etc., the water supply is not only 

 unlimited, but the water is constantly brought into contact with 

 the epidermal cells. In soils, on the contrary, the amount of 

 available water is usually limited. ^loreover, soil water moves 

 much less readily, and consequently the epidermal cells must 

 themselves move toward the water. They accomplish this by 

 extending their surface in the form of a long, narrow hair. 



While the shape and position of roots are largely determined 

 by water content and other soil factors, most roots possess the 

 common property of growing in the direction of the greatest 

 moisture. This property is termed hydrotropism, i.e., a turning 

 toward water. It is not only possible to demonstrate by experi- 

 ment that the growth of root-hairs is in the direction of the great- 

 est water content, but it can also be shown that the root actually 

 curves toward moisture. The curvature takes place in the region 

 of greater growth, i.e., at a place some distance behind the tip. 

 The stimulus, however, seems to be received by the tip, and is 

 then transmitted to the region that is growing most rapidly. 

 The advantages of hydrotropism are evident, inasmuch as it 

 enables the plant practically to go in search of water at those 

 times when the supply is more or less inadequate. 



The extent of the root system differs widely in different plants. 

 In some species the root soon stops growing, and in consequence 

 always remains poorly developed; in others the root system 

 grows throughout the life of the plant or at least for a long time. 

 Plants that grow in deep soil usually have more extensive root 

 systems than those in shallow soils, but there arc many excep- 

 tions to this, ^loreover, the prevalent opinion that there is a 



