i 9 i6] ROBERTS— EPIDERMAL CELLS OF ROOTS 49 l 



sativum he finds that the protrusion of the root hair takes 

 place opposite the nucleus. KiiSTER'(i2) 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 Spergnla 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 90 per cent salt 

 water, Suaeda maritima 60-70 per cent, and Atriplex pabulum 

 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. Fitting (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 o. 20 n to 0.30 fj.. 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 KN0 3 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 (9) 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- 



