THE STEUCTUEE OF THE BOOT 5 



brown portions of the root are useless, for the purpose of 

 absorption. 



If we examine such a root-hair from its tip to its point 

 of origin, we find it to be a slender tube filled with liquid, 

 and projecting like the finger of a glove from one of the 

 external cells of the root. These cells, which form the outer 

 layer {epidermis) (Fig. i E) of the root, and which are very 

 thin-walled in this region, have, by elongating to form root- 

 hairs, immensely increased their absorptive surface. The 

 root-hairs, therefore, enable the root to present in a very 

 small region an enormously large absorptive area. 



As the chief substance which is to be absorbed by the root 

 is water, we find that the development of the absorptive sur- 

 face of the root is often directly proportional to the need for 

 water. In the case of aquatic plants, for instance, in which 

 the entire plant is constantly in contact with water, there is 

 no need for an increase of the absorptive surface, and the 

 production of root-hairs seems unnecessary. As a matter of 

 fact, the roots of these plants are often destitute of root-hairs. 

 The roots of the water-hemlock and other plants have been 

 observed to be smooth as long as they live in water, but as 

 soon as the plants are grown in dry soil, their roots become 

 clothed with root-hairs. In typical land plants, on the other 

 hand, the necessity for an increase of the absorptive surface 

 will become greater the larger the leaves are, and the greater 

 therefore the transpiratory surface becomes. Plants, on the 

 other, hand, possessing narrow, tough leaves, which reduce the 

 transpiration, will be able to absorb sufficient water even if no 

 root-hairs are developed. This indeed is the case, and the 

 roots of various Conifers which bear needle-shaped leaves have 

 been found to be devoid of root-hairs, the thin flattened layer 

 of epidermal cells being able to absorb sufficient water for the 

 needs of the tree. Sometimes on the same plant gome roots 

 may be devoid of hairs, while others are sparingly, others 

 largely covered with root-hairs, and this will depend upon the 

 facility which the roots experience in obtaining their water 

 supply. The more difficult they find it to obtain water, the 

 more perfectly will the absorbing hairs be developed. In 

 perfectly dry soil, too, the development of root-hairs will 



