1916] ROBERTS— EPIDERMAL CELLS OF ROOTS 503 



zation of swelling takes place, which immediately follows the 

 initial swelling of wall /. 



The position of the nucleus can have nothing to do with the 

 initial swelling, as the swelling is over the entire length of the 

 wall 1. Since the weaker places bear no relation to the position 

 of the nucleus, the nuclear position can in no way affect the forma- 

 tion of the root hair. 



In this discussion the second type given by Leavitt is not con- 

 sidered, the type in which only specialized epidermal cells form 

 root hairs. Preliminary observations indicate that the osmotic 

 pressure of the short cells varies from that of the other cells, but 

 there are doubtless other factors determining the hair formation. 



Investigations so far indicate that there are two factors of 

 importance in the initial formation of root hairs. One is the 

 unequal pressure acting upon either side of wall 1; the other is 

 the variation in the physical character of the wall. 



The difference in pressures on the two sides of walls 2, 4, 6 is so 

 much less than the difference on the two sides of wall 1 that it is 

 negligible. The osmotic pressure of the root hairs of the plants 

 investigated when grown in moist air shows slight variation. In 

 the plants examined the osmotic pressure approximates 5 atmos- 

 pheres. The walls 2, 4, 6 have an opposing pressure equal to or 

 greater than 5 atmospheres, but wall / must sustain a pressure of 

 4 atmospheres, for on this wall the internal pressure is opposed by 

 only one atmosphere, when the root is grown in moist air. This 

 pressure is sufficient to account for the initial swelling of wall 1. 



The result of increasing the osmotic pressure on the outside of 

 wall 1 by growing radish seedlings in sucrose solutions of increasing 

 osmotic value proved that the opposing pressures on either side of 

 wall 1 still maintained a balance of at least 4 atmospheres in favor 

 of an outward pressure. 



Root hair formation is retarded when the moisture content of 

 the air is decreased. This is shown by the experiments with seed- 

 lings grown in Petri dishes in alternating dry and moist air, and 

 in those grown over sulphuric acid solutions. Reduced moisture 

 affects the membranes by decreasing the extensibility of the walls 

 due to an increase in the elasticity of the colloids. The osmotic 



