60 BIOLOGY FOR BEGINNERS 



For the purpose of absorbing as much as possible, the surface 

 of the active parts of all roots is covered with root hairs. These 

 are outgrowths of the epidermal cell walls and increase the total 

 absorbing surface enormously. They also enable the osmotic 

 membrane to actually touch the film of water, which, even in the 

 driest soils, clings close to the soil grains. 



So important are these root hairs that their injury or loss might 

 mean death to the plant, hence they are never borne at the extreme 

 tip of the root, where its growth through the soil would strip them 

 off, but are found a little back from the tip and extending various 

 distances along the younger roots. 



As the root grows, new hairs are produced near the tip, to gather 

 moisture from new areas; the upper ones die away; the cortex 

 and epidermis thicken, cease active absorption, and become 

 protective in use. In frequent cases, the root hairs secrete a weak 

 acid which helps in dissolving soil substances and in penetrating 

 hard earth. 



The adaptations of root hairs may be summarized as follows: 



1. Extent of surface. 2. Thinness of walls. 



3. Protection from injury. 4. Location. 



5. Close contact with soil grains. 6. Acid secretion. 



Geotropism. In order that roots may always grow where they 

 can best absorb food materials, they show a tendency always to 

 grow downward, i.e., toward the earth. This might at first thought 

 be credited to mere weight, but it is evident that stems, though 

 equally heavy, cannot be made to grow down, and that roots, 

 though lighter than the soil, still force their way through it, and 

 cannot be made to grow upward, even though repeatedly started 

 in that direction. 



This turning of roots and stems is caused by the attraction 

 of the earth, called gravitation, and this response that plants 

 make to gravitation is called geotropism positive in the case of 

 roots, and negative in the case of stems. Positive geotropism 

 plays an essential part in absorption by causing the roots to pene- 

 trate the soil rather than grow in any chance direction. 



