1916] ROBERTS—EPIDERMAL CELLS OF ROOTS 491 
sativum he finds that the protrusion of the root hair takes 
place opposite the nucleus. Kiister (12) takes exception to this. 
In the figures in SNow’s (21) work the nucleus has no definite 
position. 
OSMOTIC PRESSURE.—PFEFFER (18) found the osmotic pressure 
in corn root hairs to be greater than that of the cortical cells. 
STIEHR (23) found that root hairs on seedlings of Spergula which 
had grown in moist air when put in a 1 per cent magnesium solu- 
tion burst at once, the nucleus being thrown out, and that always 
a greater percentage of the younger hairs burst. GANONG (7) 
found that the root hairs of Salicornia withstood go per cent salt 
water, Suaeda maritima 60-70 per cent, and Afriplex patulum 
40 per cent. DRABBLE and LAKE (3) found that in mesophyll 
cells in plants growing in the same condition the osmotic pressure 
of the cell sap is generally the same, and in the plants of any area 
the osmotic pressure varies with the physiological scarcity of 
water. Frrrinc (6), using cells from the leaf, found that species 
showing high pressures in dry desert conditions show much lower 
pressures in moist situations, and concluded that certain plants 
adjust their osmotic pressure to the medium. EcKerson (4) 
found that the root hairs were plasmolyzed by sucrose, varying 
from 0.20 u to 0.30 u. STANGE (22) found that in water cultures 
with nutrient solutions of high concentrations the osmotic pres- 
sures of roots are much higher than others; bean and pea in moist 
soil have pressures of 6.25N KNO, when growing in concentrated 
medium. 
MemBRANES.—SCHWARz (20) found that the membrane of the 
root hair of Taxus baccata was of two parts: an inner layer which 
stains blue with chlorzinc iodide, and an outer layer which stains 
yellow brown. This outer layer is a mucilage which is hard in 
dry soil, and as moisture increases strongly swells and finally goes 
into solution. The mucilage layer gives no color with iodine and 
sulphuric acid, but stains red in an alcoholic solution of acid 
Carmine. Hesse (g) measured the thickness of the membranes 
of the root hairs of many plants. He found that the thickness 
varied greatly with the family, but is nearly uniform within the 
family; that in any plant the thickness of any root hair mem- 
