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 (iig. 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 in 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 l>ecause 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. Roots, 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 readity. Even rather small 

 root branches are covered with root bark. But the smallest 



