THE PLANT AS A WORKING MACHINE 9 
7. Roots. In places such as very steep hillsides or along 
river banks one may often see plants whose roots have been 
uncovered by removal of the soil in which they grew. When 
the soil is removed, the roots sooner or later are unable to 
hold the rest of the plant in place, and it may fall to the 
ground (fig. 4). When their roots are partially uncovered, 
trees that have withstood heavy winds for years may suc- 
cumb to winds that are less severe than many which they 
had previously been able to withstand. In cultivated hilly 
fields heavy rains often erode the soil from above the roots 
to an extent that allows the plants to fall. Obviously one 
function served by roots is that of holding plants in place; 
this is called anchorage. 
But oftentimes one may observe plants whose roots are 
partially uncovered, the plant still erect im position but wilt- 
ing or with yellow leaves and evidently not growing well. 
Furthermore, in some cases the roots on one ‘side have 
been uncovered and the plant has fallen, but its branches 
and leaves are still in a thrifty condition. If all or nearly all 
the roots are exposed, or if the soil is extremely dry, the water 
supply of the plant, which comes through the roots, is inter- 
fered with, and because of a shortage of water the leaves may 
wilt: but if a part of the roots are still well imbedded in 
moist soil, though the stem and branches may have fallen, a 
fairly adequate supply of water may still reach the stem and 
leaves, and wilting and death may not follow. Itoots, there- 
fore, serve not only for anchorage but also as structures 
through which the plant receives its water supply, that is, as 
organs of water absorption. 
8. Rootlets and root hairs. In examining the root system 
of any common plant (fig. 3) it is usually seen that the roots 
directly joined to the stem are relatively few and large, and 
that they divide and redivide until extremely fine rootlets are 
formed. In most plants the larger roots are covered by bark, 
through which water does not pass readily. Even rather small 
root branches are covered with root bark. But the smallest 
