ioio] 



ROBERTS— EPIDERMAL CELLS OF ROOTS 



499 



hairs has without the calcium pectate membrane on side i of the 

 epidermal cell a third thin layer which is of cutin. It stains red 

 with Sudan III and is insoluble in 50 per cent chromic acid. This 

 wall 1 of the epidermal cell of the stem is then chemically different 

 and physically of greater rigidity. 



c) The osmotic pressure on both sides of walls 2, 4, 6, and on inside 

 of wall 1. — The cortical cell is always plasmolyzed by a solution of 

 sucrose which is 0.02M-0.04M stronger than that which plas- 

 molyzes the epidermal cell; this indicates that on the outside of the 

 wall 6 there is a greater pressure than on the inside; whereas on the 

 outside of 2 and 4 there is a pressure equal to that on the inside. 

 On the outside of wall / there is a pressure of one atmosphere. 

 The variation between the outside and inside pressures of wall 6 

 is small, but there is a great variation in the pressures upon the two 

 sides of wall 7. Table IV gives the osmotic pressure of the root 

 hairs of different plants grown under similar conditions. 



It will be noted in table IV that all are plasmolyzed by a solu- 

 tion of sucrose between 0.22M-0.30M sucrose, except barley. 

 This is the equivalent of 5. 10-7. 70 atmospheres. Two facts are 

 indicated here: first, that in moist air there is a minimum of differ- 

 ence of 4 atmospheres between the pressure upon the inside and 

 upon the outside of wall /; and second, that under similar conditions 

 there is a remarkably slight variation in the osmotic pressure of 

 root hairs of seedlings of different genera. Some roots respond 

 more readily than others to the percentages of moisture present 

 in the media. Cabbage, alfalfa, squash, and corn show a definite 



