264 PLANT PHYSIOLOGY 



than water can be supplied from below, results in the negative 

 pressures observed. If atmospheric pressure were really efficient, 

 such differences would not exist, and while atmospheric pressure 

 may assist to a slight extent in causing some of the movement of 

 water from cell to cell in the root, it is completely inadequate to 

 furnish an " explanation." 



Root Pressure. — Root pressure, — the same force which re- 

 sults in bleeding as described in the last chapter, — has also been 

 invoked to explain the ascent of sap. This force, first noticed by 

 Hales (1735) in a grapevine, is sufficient to burst a bladder tied to 

 the end of the stem and will, under favorable conditions, cause a 

 pressure of 1.5 atmospheres. This pressure, which is the result of 

 turgor in the cells below, might be of some assistance were it 

 present when needed, but unfortunately root pressure is greatest 

 when there is no transpiration and when it is consequently of least 

 use. When the leaves are actively transpiring, little or no root 

 pressure can be detected. Under the conditions that result in 

 bleeding, namely when the top of the plant is removed, it becomes 

 most manifest. This pressure from below could never, therefore, 

 be more than an aid and could not in any sense be the real cause 

 of the ascent of sap. Barnes (1910) denied even this and said: 



If a boy could push a wagon while the horse walked, he would be un- 

 able to push as soon as the horse's speed exceeded his own. If he clung to 

 the wagon, he would be merely a drag, though if he ran he would be less 

 of a drag than if he made no exertion. The transpiration horse often 

 goes too fast with the water wagon for the root pressure boy to push. 

 Then his grip is broken at once and he is no drag on the load, for root 

 pressure cannot even hold on like the boy and "help" by not being wholly 

 a drag. 



This, however, does not seem to be the correct figure. The root- 

 pressure boy does not run along behind the water wagon. In- 

 stead he stands by the side of the railroad track while the cars go 

 by and gives each car a little push as it passes. While he does not 

 move the train he may help a little. At any rate he does not hinder 

 its passage or act as a "drag." 



A similar pumping action on the part of the ray and other living 

 parenchyma cells scattered along the way between the roots and 

 the leaves has been offered by some as an explanation of the rise 

 of sap. Bose (1922) places these cells in the root and stem next 

 to the endodermis and considers that the xylem acts like a reservoir 



