160 COLLEGE BOTANY 



growth of the p'laiit. However, these processes are accom- 

 panied by a movement of the water, due to the force of capillar- 

 ity, from, the lower layers of soil towards the upper or surface 

 layers. Therefore, the plant is continually receiving a supply of 

 water from below. During periods of drouth this supply is not 

 sufficient for the normal growth and development of the plant. 



Relation of the Root-hairs to the Soil and Water. — The 

 structure of the roots is such that they do not take water 

 directly. This function is performed by the roO'1>hairs which are 

 borne in a definite zone just back of the growing tip. They exist 

 for a short time only and then disintegrate; new ones being 

 formed as the root increases in length (page 65). The root- 

 hairs are interwoven among the soil particles. They vary in 

 number with the gro'wth of the plant and the amoant of avail- 

 able water. The actively growing plants need a large amount 

 of water and must have the necessary number of root-hairs to 

 secure this water supply. The soil water contains in solution 

 many minerals which are necessary for the growth of the plant, 

 some which are unnecessary, and, sometimes, some which are 

 injurious (see Chapter XVII). 



The movement of the dissolved minerals into the root hairs 

 and thence into the roots depends primarily on the selective 

 character of the permeable membrane, the density of the solution 

 in the root-hair and the density of the soil solution. The amount 

 of mineral salts entering the plant does not depend on the amount 

 of water absorbed. 



Imbibition. — The power to take up- water is not restricted to 

 living organisms. Blocks of dead wood will absorb water by 

 imbibition, which is the power to talie in and hold water by capil- 

 larity or surface tension. Blocks of dry wood which have talten 



