146 



PHYSIOLOGY OF NUTRITION 



the two rates become equal, the two curves become coincident, and water 

 is then moving through the piece at a uniform rate throughout. Similar 

 experiments with tubes filled with sand containing air and impregnated with 

 water gave concordant results with those obtained with the pieces of stem. 

 Votchal conceives that air bubbles in the wood act simply as resilient springs 



that transmit and distribute the thrust 

 imparted to them more slowly and evenly 

 than would a continuous homogeneous 

 water column. The effective forces 

 applied at the ends of the conducting 

 channels — i.e., the force of foliar trans- 

 piration and that of root pressure- 

 furnish energy to account for the ascend- 

 ing water current in plants. Root pressure, 

 produced by osmotic forces, exerts a pres- 

 sure upon one end of the water column in 



Fig. 83. — Arrangement to show rise of a 

 mercury column caused by evaporation of 

 water from the leaves of a cut twig. 



Fig. 84. — Evaporation of water through 

 a membrane, causing rise of mercury in 

 tube below. 



the wood, while evaporation of water from the leaves establishes traction at 

 the opposite end.* 



8 Root pressure is not to be considered as generally important in the ascent of water through 

 plant stems. The mere existence of "negative gas pressure" in the vessels shows that the 

 liquid above the gas bubbles is not being forced upward by a pressure applied below. Perhaps 

 the simplest argument in favor of dismissing root pressure from consideration in the general 

 problem of rise of sap lies in the fact that this pressure is found to be highest when water 

 movement is slowest and lowest when movement is most rapid.— Ed. 



