THE PLANT COVERING OF THE EARTH 325 



quarter of an inch from the tip and extends back as much as 

 an inch or more. Each hair is merely a tubelike extension of 

 a cell upon the surface of the 

 root (fig. 158), and it 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 in- 

 creased in this way. A single 

 root hair is veiy small, but 

 there are thousands of them 

 on even a very short piece of 

 root (1314 to -j-J-g. square inch 

 have been found on pea roots). 

 They extend 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 



159. Ascent of water 



Longitudinal section of a stem show- 

 ing route of ascent of water; b, bark; 

 .. sapwood ; h, heartwood ; p, pith. 



Ordinarily Spread ^'ater ascends principally through the 



sapwood 



much farther underground 

 than its branches do aboveground. Not only do the roots 

 spread widely, but by repeated branching they are divided 

 into thousands of small rootlets, which penetrate so thor- 

 oughly to all parts of the soil that it is often difficult to find 



