Lecture 4 — 83 — Movement and Distribution 



sugar. This was less important for the initially high- 

 sugar plants than for the initially low-sugar plants. 

 Furthermore, the largest increment of growth during 

 the experimental period was made by the initially 

 low-sugar plants exposed to illumination and low 

 humidity. 



Experiments on Exudation and Root Pressure : — I 



shall now turn to a special phase of salt movement 

 that has some relation to certain of the experiments I 

 have already described. I refer to the phenomena 

 of which root pressure is a manifestation. Some- 

 thing of the general nature of this physiological 

 response has been known for a long time, but its 

 causes and significance are still frequently the subject 

 of debate. In many young plants — barley or cucurbit 

 plants are good examples — conditions of root pressure 

 are made evident when transpiration is suppressed, 

 by bleeding from cut stems, or by guttation, the 

 extrusion of drops of liquid from the tips or margins 

 of leaves. The study of the bleeding sap that orig- 

 inates in the conducting vessels of the xylem is of 

 considerable interest for our present inquiry. This 

 sap, as several investigators have found, can attain 

 a relatively high concentration of inorganic solutes, 

 higher than their concentration in the external nu- 

 trient solution. This may also be true of the gut- 

 tation fluid, which is often far from being pure water, 

 as is sometimes assumed. 



More than one cause of root pressure can, per- 

 haps, be effective but in young barley plants a definite 

 interrelation was found in our experiments* between 

 the absorption and movement of salt and bleeding or 

 guttation. If the roots are immersed in distilled 

 water, even though ample aeration is provided, gut- 

 tation will soon cease or become slight in rate. If 

 the roots are placed in a dilute salt solution without 



*T. C. Broyer and D. R. Hoagland, unpublished. 



