44 



PLANT BIOLOGY 



whole outer surface of young roots is covered with a single 

 layer of thin-walled cells which form the epidermis. Many of 

 these cells develop tubular outgrowths known as root-hairs 



FIG. 11. Cross section of root, FIG. 12. Diagram of a lengthwise 



showing root-hairs and epidermis section of two root-hairs with 



cells on the outer surface, cells of adjacent cells of the epidermis, 



cortex within, and woody central and with two cells of the cortex, 

 cylinder with its ducts. (Bailey.) 



(see 56, B). By studying Fig. 12 it will be evident that each 

 root-hair consists of a cell-wall lined by a thin layer of 

 protoplasm. The interior of the cell is largely filled with 



cell-sap. On the outside 

 of each root-hair is soil- 

 water. All the conditions 

 necessary for osmosis are 



FIG. 13. Portion of a root-hair with ad- 

 hering particles of soil. (Strasburger.) 



therefore present. The 

 cell-wall is the membrane 

 which separates the soil-water from the denser cell-sap. 

 From the law of osmosis, we should expect a flow of liquids 

 in both directions, the greater flow being into the cell-sap 

 from the soil-water. It has been found, however, that the 



