114 THE ELEMENTS OF VEGETABLE HISTOLOGY 



structures are located just above the root cap and form 

 a narrow zone between this and the parts of the root 

 covered with epidermis (Fig. 44, No. 1). They are 

 found only on the smallest branches of the root and, 

 although the absorbing surface of each hair is very 

 small, in the aggregate they present an absorbing sur- 

 face as large and widespread as that of the leaves. 

 Root hairs are similar to trichomes in derivation and 

 are therefore outgrowths of the epidermal cells. Each 

 is in the form of a long, slender, finger-like projection 

 from an epidermal cell. Their walls are extremely 

 thin, and it is possible that the acid reaction, which 

 is apparent when root hairs come into contact with 

 moist litmus paper, is due to an acid secretion. An 

 acid secretion in the vicinity of the root hairs would 

 be very desirable because it would convert many 

 insoluble soil constituents into soluble compounds 

 which are dialyzable. The raw materials consisting 

 of water with dissolved salts dialyze through the thin 

 walls of the root hairs. They eventually find their 

 way into tubes or vessels through which they are 

 transported to other parts of the plant to be manu- 

 factured into assimilable substances. The walls of 

 root hairs are composed of cellulose and may show 

 slight coloration. In the living plant, root hair cells 

 are always in a region of active growth and therefore 

 protoplasmic cell contents are present. 



In certain instances the work of absorbing liquids 

 may be performed by plant organs other than root 

 hairs. Plants of parasitic habit and those growing 

 in desert regions show particular adaptability as regards 

 means of absorbing water. Parasitic plants, or those 

 which gain their nutrients by burrowing into the tis- 



