274 



ABSORPTION OF WATER 



hand, produce root hairs abundantly whether they develop in the soil or in 

 water. 



A root hair is essentially a tubular outgrowth of the peripheral wall of 

 an epidermal cell, closed at its distal extremity, projecting more or less at 

 right angles from the long axis of the epidermal cell of which it is an inte- 

 gral part (Fig. 72). Not all of the epidermal cells produce root hairs 

 Root hairs may vary from less than a millimeter up to about a centimeter in 

 length. On many seedlings several hundred root hairs may be borne on a 

 square millimeter of root surface. On some herbaceous species on which 



such measurements have been 

 made the presence of root 

 hairs on a given area of root 

 increases the exposed surface of 

 that area from 5 to 20 fold. 



The cell wall of a root hair 

 is thin and delicate, and is con- 

 structed principally of cellulose 

 and pectic compounds. The 

 outer lamella of the wall seems 

 to be composed entirely of 

 pectic compounds. The tenacity 

 with which root hairs usually 

 adhere to the soil particles with 

 which they are in contact is due 

 to this pectic coating. Due to 

 this intimate contact between the root hairs and soil particles it is difficult 

 to separate the two by washing the root tip or by any other means without 

 severely injuring or destroying most of the root hairs. The inner wall of a 

 root hair is lined with a thin layer of cytoplasm which is continuous with the 

 cytoplasm of the epidermal cell of which the root hair is a part. In water 

 or in moist air the root hairs are usually straight, but in the soil they are more 

 or less contorted, conforming to the shape and distribution of the soil particles 

 among which they penetrate. 



The Pathway of Water through the Root. — Water enters the roots 

 through the walls of the root hairs and epidermal cells of the root tip. It 

 then passes through successive rows of the thin-walled cortical cells, and 

 then through the cells of the endodermis (Fig. 70). 



The structure of the walls of the endodermal cells is peculiar. Two main 

 types of such cells have been recognized. In one type (Fig. 73, J) the inner 

 tangential and radial wall, or sometimes the entire wall, is thickened. These 



Fig. 72. A young root hair. 



