USE OF SOIL MOISTURE BY PLANTS 109 



plant throughout the season. A rapidly growing plant, 

 for example, may use daily a very large quantity of. water 

 but only for a relatively short time, while a more slowly 

 growing plant, using daily a smal er quantity of water, 

 but for a longer period of time, may in the end use much 

 more water. The rate at which a plant uses water refers 

 invariably to the quantity used per hour, day or any other 

 unit of time, during certain periods of its growth, and is 

 not invariably a measure of the total water-needs of the 

 crop. 



77. Absorption of water by roots. The roots are the 

 organs of water-absorption. Practically no water is taken 

 into the plants by the stems or leaves even under con- 

 ditions of heavy rainfall. In the absorption of water 

 from the soil, the young roots are most active, and, of 

 these, only certain parts are actively engaged in water- 

 absorption. At the tips of the young roots are numerous 

 fine hairs, known as root-hairs, clustering near the tip of 

 the root. These are the organs of the plant that absorb 

 soil water. As the root-hairs grow older, they lose their 

 power of water-absorption; in fact, they are active only 

 when they are in actual process of growth. Water-absorp- 

 tion, therefore, occurs near the tips of the growing roots, 

 and, whenever the plant ceases to grow, water-absorption 

 also ceases. 



The root-hairs are filled with a solution of various 

 substances, as yet poorly understood, which play an 

 important part in the absorption from the soil of water 

 and plant-food. Owing to their minuteness, the root- 

 hairs are in most cases immersed in the moisture film that 

 surrounds the soil particles, and the soil moisture is taken 

 directly into the roots from this film by the process of 

 osmosis. Without entering into a discussion of the com- 



