THE MECHANISM OF ROOT PRESSURE AND ITS RELATION 
TO SAP FLOWi 
James Bertram Overton 
(Received for publication January 21, 192 1) 
The important role played by osmotic pressure in plants is well recog- 
nized. Plant physiologists assume that osmotic pressure in the peripheral 
cells of the root is in some way connected with the process by which sap 
is continuously supplied to the xylem vessels, and the complex conditions 
controlling the movement of water from the soil through the root to the 
vessels are passed over by the use of the phrase "root pressure." Most of 
us are familiar with the contributions of Dixon, supported by the rather 
brilliant work of Renner, that water passes through the roots to the leaves 
because the leaves tend to dry out due to water being abstracted from them 
by evaporation, resulting in the water in the vessels passing into a state 
of tension, which is transmitted equally in all directions. This condition 
obtains unless root pressure or atmospheric pressure or both is forcing water 
up the stem more rapidly than it evaporates. 
The cohesion theory of sap flow has received much attention and has 
been supported by considerable evidence. Transpiration or growth ap- 
pears to produce an increase in the "saturation deficit" (Renner) or "in- 
cipient drying" (Livingston) in the exposed leaf cell walls which is followed 
by a corresponding increase in the saturation deficit in all the cells abutting 
upon the intercellular spaces of the Ifeaf. Dixon points out that this con- 
dition is a sufficient cause for the entrance of water into the root and its 
passage through the root periphery. Renner has shown that the water- 
absorbing power of the root is directly related to the saturation deficit in 
the leaves above, and that the root's absorbing power appears to be directly 
referable to the tendency of the exposed cell walls of the root to dry out 
on account of passage of water to other parts of the plant. According to 
his view, during transpiration the root is rendered flaccid and therefore 
able to absorb water, although root pressure and passive absorption may 
under certain conditions work in combination. 
Dixon holds that the entry of water into the root depends upon the 
gradient of pressure as we pass from the outside of the root to the inside of 
the tracheae, there being a fall of pressure due to the continuous water all 
the way up the stem to the leaves; thus the flow of water up the highest 
trees is due to the evaporation and condensation produced by the difference 
between the vapor pressure in the soil spaces and that obtaining around the 
^ Invitation paper read before the Physiological Section of the Botanical Society of 
America, in the symposium on biophysics, at Chicago, December 28, 1920. 
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