70 



PRACTICAL COURSE IN BOTANY 



strong affinity for water, and its presence or absence has a 

 marked influence on their direction of growth, being often 

 sufficient to overcome that of geotropism (Exp. 47). There 

 are many trees and shrubs, notably willow, sweet bay, red 

 birch, and the like, that grow best on the banks of streams 

 and ponds, where their roots can have direct access to water. 

 Excess of moisture, however, is injurious to most land plants 

 by preventing the roots from getting sufficient air for res- 

 piration. 



72. The conditions of absorption. — The sap in the root 

 cells is normally denser than the water in the soil, so there is 

 a continuous flow from the latter to the former. But if, 

 for any reason, the density of the liquids should be reversed, 

 the flow would set in the opposite direction, and if continued 

 long enough, the strength of the plant would be literally 

 " sapped " by the exhaustion of its tissues, so that it would 

 die. What is this process of cell exhaustion called ? 



73. The use of acid secretions to the root. — It was 

 shown in Exp. 50 that carbon dioxide and probably other sub- 

 stances occur in the im- 

 mediate vicinity of roots. 

 Carbon dioxide is an ac- 

 tive agent in dissolving 

 the various mineral mat- 

 ters contained in the soil, 

 and as these last can be 

 absorbed only in a liquid 

 or a gaseous state (63), 

 the advantage to the 

 root as an absorbent or- 

 gan, of being able to se- 

 crete such active sol- 

 vents, is obvious. 



74. Relation of roots 

 x^ OK A . 1 . . u- to the soil. — In order to 



Fig. 85. — A natural root etching, 



found on a piece of slate. perform their work of ab- 



