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 Helzan- 
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 : 
1. 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. 
