504 BOTANICAL GAZETTE [December 



pressure of the cell content is also increased by the modification of 

 the moisture content of the air, but this is less effective because of 

 the greater effect of the reduced moisture content upon the char- 

 acter of the wall. 



The effect of reduced moisture content is again demonstrated 

 in the experiments with differential media. In the case of Helian- 

 thus the effect of the sucrose molecule upon the colloids of the 

 membrane and in the radish of glucose is to decrease the moisture 

 content and the extensibility of the wall. 



The wall is found to be composed of an outer membrane of 

 calcium pectate and an inner membrane of cellulose. The only 

 exception so far is in corn. A partial third membrane of callose is 

 present in some. The amount of callose is so small and so little 

 is known of its chemical nature that for the present it seems unim- 

 portant. The cellulose layer is important in that it gives greater 

 strength to the wall. 



The pectin layer and also the pectic substances within the cell 

 are very important in the absorption and the retention of water, 

 since pectin is a hydrophyllous colloid; and for this reason must 

 influence the development of root hairs. The cations and the 

 anions of the mineral substances in solution increase or decrease 

 the absorptive capacity of this membrane according to a definite 

 known manner, as indicated in the lyotropic series, investigations 

 upon which should be made. 



Summary 



i. The initial formation of the root hair is indicated by a general 

 swelling of the outer wall of the epidermal cell. 



2. The swelling is produced if the physical resistance of the 

 wall is overbalanced by the higher osmotic pressure which is main- 

 tained on the inside of the wall. 



3. Further swelling followed by growth takes place at the less 

 resistant portion of the wall. 



4. This region bears no relation to the position of the nucleus. 



5. The wall of the root hair is composed of two parts, an inner 

 membrane of cellulose and an outer membrane of calcium pectate. 



