IQI6] 



ROBERTS— EPIDERMAL CELLS OF ROOTS 



501 



c) The osmotic pressure on both sides of r, 2, 4, 6.— The only wall 

 on which the osmotic pressure may be made to vary at will is on 

 the outside of wall 1. Radish seedlings were grown in sucrose 

 solutions of increasing concentrations in order to vary the osmotic 

 pressure on the outer wall. The results obtained as the average 

 of several series are given in table VI. 



Diameter root 

 in 1/100 mm. 



Length hair i: 

 1/100 mm. 



Osmotic 

 pressure 



Difference 

 between osmotic 

 pressure of root 

 hair and medium 



Air 



Water 



0.02 molecular sucrose 



0.04 " " 



0.06 " " 



0.08 " 



0.10 " " 



0.12 " " 



0.14 " " 



0.16 " " 



0.18 " " . 



o . 20 " " 



0.22 " " 



o . 24 " " 



o . 26 " " 



0.28 " " 



0.30 



0.32 " 



0.34 



0.36 " 



0.38 



o . 40 " 



0.42 " 



0.44 



0.48 " " 



o 48 " " 



0.52 " 



o . 60 " 



0.65 " 



48 



QO 

 IOO 



It is seen that as the concentration of the media increases the 

 osmotic pressure of the root hair increases almost in direct propor- 

 tion. Thus the root hair maintains an osmotic pressure from 4 to 6 

 atmospheres above that of the medium. 



Differential media. — The effect of different media on the two 

 sides of a seedling root was determined. Petri dishes were filled 

 with the solutions, seeds were attached to the edge, and the roots 



