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 xu). 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 roots 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 #, 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 
