10 



INTRODUCTION TO BOTANY 



and terminal rootlets have no bark, and from their surfaces 

 there grow very fine tubular threads known as root hairs 

 (figs. 5 and 6). Root hairs have extremely thin walls., through 

 which, water from the soil can pass into the interior ; thence it 

 passes upward through rootlets and larger roots, through the 

 stem and into the leaves. Substances that are in solution in 

 the soil water may be transferred into the plant through the 



delicate walls of the root hairs and 

 their lining. Root^Jiairs do not 

 grow over all the surface of the 

 terminal rootlets, but develop a, 

 little way from the root tip. They 

 do not live very long % the older 

 ones constantly dying. Thus, as 

 the root tip grows forward through 

 the soil the actual number of root 

 hairs on a rootlet may remain prac- 

 tically constant during the growing 

 season, on account of the dying of 

 older root hairs and the develop- 

 ment of new ones near the root tip 

 on the new growth of the rootlet. 

 It is evident that the area of root 

 hairs advances, although no indi- 

 vidual root hairs move forward 

 through the soil. Although ex- 

 tremely delicate in structure, root 

 hairs grow between and around the hard particles of soil 

 (fig. 6). It is easy to count the root hairs on a small portion 

 of the root. Corn rootlets grown in damp air have been found 

 to bear 425 root hairs on an area ^ inch square. The large 

 number of rootlets and the enormous number of root hairs 

 serve to make a network which completely permeates the soil 

 in the region of the rootlets. The root hairs are the chief 

 organs by which water and substances in solution in water 

 are absorbed from the soil. 



FIG. 5. A mustard seedling 



grown in a band of filter paper 



inside a drinking glass, so as to 



show the root hairs 



Note the difference in length and 



condition of the root hairs on the 



different parts of the root 



