40 ESSENTIALS OF BOTANY 



at any rate, can thrive very well on ordinary lake, river, or 

 well water, together with the food which they absorb from 

 the air (Chapter XIII). Just how much water some kinds 

 of plants give off (and therefore absorb) per day will be 

 discussed when the uses of the leaf are studied. For the 

 present it is sufficient to state that even an annual plant 

 during its lifetime absorbs through the roots very many 

 times its own weight of water. Grasses have been known 

 to take in their weight of water in every twenty-four hours 

 of warm, dry weather. This absorption takes place mainly 

 through the root-hairs, which the student has examined 

 as they occur in the seedling plant, and which are found 

 thickly clothing the younger and more rapidly 'growing 

 parts of the Toots of mature plants. Some idea of their 

 abundance may be gathered from the fact that on a rootlet 

 of corn grown in a damp atmosphere, and about j^y inch in 

 diameter, 480 root-hairs have been counted on each hun- 

 dredth of an inch in length. The walls of the root-hairs are 

 extremely thin, and they are free from any holes or pores 

 which can be seen even by the highest power of the micro- 

 scope, yet the water of the soil penetrates very rapidly to 

 the interior of the root-hairs. The soil-water brings with 

 it all the substances which it can dissolve from the earth 

 about the plant; and the closeness with which the root- 

 hairs cling to the particles of soil, as shown in Fig. 19, 

 must cause the water which is absorbed to contain more 

 foreign matter than underground water in general does, 

 particularly since the roots give off enough weak acid 

 from their surface to corrode the surface of stones which 

 they enfold or cover. 



52. Movements of Young Roots. — The fact that roots 

 usually grow downward is so familiar that we do not 



