WATER ABSORPTION 



47 





which the roots gathered. The factor which is effective 



in these cases cannot be easily identified, for the humus soil 



differs from the sand in many respects ; it may modify and 



attract root systems either 



because of its greater water 



content, or its greater supply 



of nutrients ; its p^ value is 



also different from that of 



sand. 



A particularly interesting 

 case is figured by Weaver 

 (1919), in a root system of 

 Kiihnia gliitinosa growing in a 

 prairie soil of stratified sands 

 and clays (Fig. 5). In two 

 beds of clay lying 8 and 13 ft. 

 deep the number of laterals is 

 very much greater than in the 

 sand. The clay is much wetter 

 than the sand, but in this 

 case, too, we may be inclined 

 to suspect that the influence of send , 

 salt supply is effective. 



§ 4. The Absorption of 

 Water 



Root Hairs. — The absorp- 

 tion of water is only to a very 

 limited extent carried on 

 actually by the roots. These 

 traverse the soil thoroughly Fig. 5.— Root system of Kuhnia 



and often extensively, but they glutinosa growing in alternate 

 ., ., , •' , , ■' layers of clay and sand. (From 



provide neither the large ab- Weaver.) 



sorbing area nor the intimate 



contact with the water films which are necessary in ordinary 



soils. Absorption is normally the work of the root hairs. 



These occupy a region a very few centimetres long, extending 



