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AMERICAN JOURNAL OF BOTANY 
[Vol. 8. 
leaves, and the tensile water flows under the action of this difference from 
end to end of the plant. The tensile stress transmitted to the root has the 
ultimate effect of drying out the root surface, and the gradient of pressure, 
which causes the passage of water from the soil into the root, is referable 
to the concave menisci formed in the cell walls of the root periphery due to 
the tension in the sap. 
According to the above-outlined picture it would appear that in the 
absence of root pressure the passage of water from the soil into the root 
periphery is purely passive, the root acting merely as a filter in water intake, 
a view, however, to which Jost raises objections, although it has been shown 
that much water may be taken in by dead roots and that roots of transpiring 
plants can absorb water from solutions of high osmotic pressures. Con- 
cerning the mechanics of root absorption, however, we know comparatively 
little on account of the difficulties of observation, and little is actually 
known about the forces which are operative in the passage of water from 
the soil through the root into the vessels in sufficient quantity to supply the 
transpiration needs. 
Root pressure is not generally considered as important in causing the 
ascent of sap through the stem. The presence of a negative gas pressure 
in the vessels shows that the water is not being forced up from below, and 
root pressure is lowest when transpiration is most active and highest when 
water movement in the stem is slowest. Dixon regards the function of 
root pressure to be a periodic flooding of the vessels with water, tending to 
bring the gas bubbles into solution and to reestablish conditions for tension 
throughout the water tracts, the influence of root pressure in sap flow being 
an indirect one. The importance of osmotic phenomena in the root as 
far as water is concerned remains to be determined. It has been suggested 
by Livingston and Pulling that root pressure is mainly effective in main- 
taining the form of the roots and their contact with the water films of the 
surrounding soil. It has been shown that the rate of water absorption by 
roots is not proportional to the osmotic pressure of the root cells and that 
they can operate equally well with high or low turgidity so long as they 
are not deformed. 
The exudation of liquid water from passive hydathodes, such as in 
Colocasia, and the secretion of water and solutes from wounds, show that 
the roots of many plants under certain conditions can develop considerable 
pressure. Whether or not it is admitted that sap pressure functions in 
sap flow, it seems certain that root cells are able io take in water and solutes 
and to pass them on to the xylem. Atkins holds that the cortical cells of 
the root have a much higher osmotic pressure than the tracheae and that 
they function as a complex semi-permeable membrane. It is evident that 
sap pressure, whether in roots or elsewhere, depends upon osmotic phenom- 
ena. It would appear from a survey of the literature that sap movement 
in roots is to be explained in the same way as the action of glands and hyda- 
