238 ELEMENTS OF GENERAL SCIENCE 



Each hair is merely a tube-like extension of a cell upon the 

 surface of the root (fig. 118), and is able to absorb water just 

 as the remainder of the cell does. A root hair usually has 

 many times as much surface as the cell from which it grows, 

 and the surface of the root is increased in this way. A single 

 root hair is very small, but there are thousands of them on 

 even a very short piece of root (1314 to T ^ square inch have 

 been found on pea roots), and they increase the surface to 

 many times what it would be without them. Also, they ex- 

 tend into the soil, from which they absorb water and in which 

 they help to anchor the plant firmly. 



The number of root tips in a single plant is ordinarily very 

 great. We commonly judge the root system by what we see 

 when a plant is pulled out of the ground, but usually .much 

 the larger part of the root system is broken off and left in the 

 ground. The roots of a tree ordinarily spread much farther 

 under the ground than its branches do above the ground. Not 

 only do the roots spread widely, but by repeated branching 

 they are divided into innumerable thousands of small rootlets, 

 which penetrate so thoroughly to all parts of the soil that 

 under a tree it is often difficult to find a cubic centimeter of 

 soil that does not contain rootlets. It is these small rootlets 

 that bear the root hairs and absorb the water. 



It must not be supposed that root hairs are roots. The 

 hairs never become roots, but, on the contrary, as the root 

 tip grows out farther the old root hairs die and new ones 

 are formed on the newer parts of the rootlet. 



254. Water-carrying tissues. We have spoken of the way 

 in which water passes from cell to cell in the tip of the rootlet, 

 and of the way in which this water is transferred from the 

 surface to the interior of the root. In some of the smaller 

 plants, such as the mosses, the water passes through the plant 

 by simple osmosis, but in all the larger plants there is a more 

 intricate system by means of which water is carried. The 

 structure of the water-carrying parts is so similar in roots 



