CHAP. III.] ROOT HAIRS 43 



clothed with a down of very fine hairs, though the last 

 quarter of an inch or so is bare. In the case of the 

 wheat, instead of a single tap-root a number of fibrous 

 roots, which branch as they become older, issue from 

 the embryo ; these likewise are clothed with root hairs 

 near the tips. By day-to-day observations we shall find 

 that the root hairs fall off as the root ages ; they occur 

 only in a region just behind the growing point of the 

 root. By carefully digging up some young seedlings 

 from the open ground or from a pot of earth, and wash- 

 ing away the soil, we shall find that the root hairs cling 

 very obstinately to some of the fine particles of soil, with 

 which they are evidently in intimate contact. A good 

 deal of the holding power of roots is due to this associa- 

 tion of the root hairs with the finest particles of the soil. 

 It is possible to show by appropriate experiments that 

 the primary root of a plant tends to grow straight down- 

 wards under the direction of the pull of gravity, but that 

 it may be deflected by its attraction for such necessaries 

 as water, air, and suitable food. The attraction exerted 

 by the supply of air is plainly to be seen in the way 

 roots of trees and any deep-rooting plants force their 

 way into drains, just as their need for air is shown by 

 the way roots will not penetrate into undrained soil, 

 but stop short as soon as the layer is reached which 

 is saturated with stagnant water and contains no air. 

 It is also found that wheat develops the best root 

 system (the foundation of a large crop later) when the 

 winter is comparatively dry, so as to leave the soil well 

 aerated ; in a wet winter or wet soil the plant neither 

 needs so extensive a root system in order to keep 

 itself supplied with water, nor is it stimulated by the 

 air to produce roots. The attraction exerted by food 

 is seen in the way a bone or other fragment of spar- 

 ingly soluble manure becomes covered and permeated 



