72 BOTANY 
a large and constant supply of water, and so must 
obtain it from the deeper regions of the soil where 
water is always present. Such plants, even though, 
like the trees of some tropical forests, they receive no 
rain for long periods together, continue to flourish, since 
they tap the accumulated supply derived from previous 
rains. 
Grasses, with their fibrous roots penetrating but 
a short distance below the surface, are best suited to 
regions where rainfall, though not copious, is well 
distributed throughout the year. As a rule, the roots 
of ordinary cultivated crops do not penetrate the soil 
to a greater depth than three or four feet, but, in dry 
regions, in their search for moisture they may travel 
down for ten or twelve feet. Lucerne growing on dry 
soil has been known to penetrate over thirty feet. 
There are various peculiarities found in the roots 
of certain plants which have arisen to adapt them to 
carry out special functions that are not ordinarily 
included in the work of this organ. The two chief 
adaptations of roots are for the storing of reserves of 
nourishment, and for climbing. In the napiform root 
of the turnip (Fig. 48), the conical root of the carrot 
(Fig. 47), and the fusiform root of the radish (Fig. 49), 
as in the case of many other biennials, reserve food 
is stored up in the first year to supply the material for 
the production of flower and fruit in the second; while 
in the tuberous roots of the dahlia (Fig. 50) a similar 
reserve supplies nourishment to the tiny shoots, that, 
in the spring, appear at the base of the old stem and 
provide for vegetative reproduction. In the ivy, the 
adventitious roots (Fig. D1) produced on the stem 
enable the plant to fix itself to walls, trees and other 
objects, and thus, in spite of its slender stem, to lift 
itself up for air and sunlight. 
We have already noted, on the roots of the bean 
and other plants of the same family, the tubercles: 
