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, 

 showing root-hairs and epidermis 

 cells on the outer surface, cells of 

 cortex within, and woody central 

 cylinder with its ducts. (Bailey.) 



FIG. 12. Diagram of a lengthwise 

 section of two root-hairs with 

 adjacent cells of the epidermis, 

 and with two cells of the cortex. 



(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.) 



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 



therefore present. The 

 cell-wall is the membrane 



