Water-Content of Plants 



51 



develop as root-hairs. It is believed that factors operat- 

 ing to increase rapidly the longitudinal extension of these 

 cells will decrease the number and length of the root-hairs. 



FIG. 14. Cross-section of a rootlet: epidermis and bases of root-hairs 

 (a), cortex (6), endodermis (c), and central cylinder (e) with xylem and 

 phloem elements. 



It would seem that in some cases the retardation of growth 

 by soil particles has a beneficial effect upon hair produc- 

 tion. In a completely saturated air or in a high temper- 

 ature - - in other words when the conditions are such as to 

 rapidly promote increase in length of the cortical cells of 



