146 • 



and previous experiments/ and no root pressure was found in plants during rapid 

 transpiration. The time of greatest transpiration seems to bear no relation to the 

 time of greatest or least root pressure, and changes in temperature that affect the 

 former do not influence the latter to any degree. 



Where no transpiration is going on the root pressure may produce sufficient 

 pressure in the plants of medium height to force the water out through the water 

 pores of the leaves, or in some cases producing blistering in the tissues of the stem, 

 as in the well-known case of the Oedema of the tomato.^ 



The relation of the root pressure with growth does not warrant any statement 

 as to the influence of one upon the other. The time of either the maximum or 

 iminimum periods of each do not correspond, and changes in temperature that af- 

 iect growth produce no changes in the constancy of the root pressure. 



Studies regarding the relation between root pressure and assimilation show 

 all negative results, and the changes producing variations in the latter have no 

 effect on the former. The same may be said of the relation between root pressure 

 and respiration. 



In view of these facts we are warranted in the following general conclusions. 



The periodicity of root pressure seems to be inherent in the plant, and has 

 either been acquired by previous adaptation to environments, or as the results of 

 the action of some constant or periodic changes in the plant. As with the 

 periodicity of growth and other periodic phenomena it does not always follow 

 that a periodic change has not been produced by some constantly or continuously 

 acting agent. 



Root pressure does not seem to have any relation to the previous periodicities 

 of the vital activities of the plant when the top was connected with the roots. 



The measure of the root pressure seems to be the osmotic activity of the root 

 hairs, and is probably due to the presence of organic acids and other substances 

 in the rhizoids that show great affinity for water. ^ 



Although the organic acids increase in the cells at 50°-60° F., yet their in- 

 crease does not seem to make any appreciable difference in the periodicity. 

 This is true even when the temperature of the soil is brought up to 55° F., ap- 

 proaching the time of minimum pressure. 



The fact that seems inexplicable is that, when the temperature is raised above 

 the point where the organic acids decompose (60° F.) *in most plants, the roots 

 may show an increase in their osmotic activity at the daily period of maximum 



^DeVries, Arb. Les. Bot. Inst. (B. I , p. 228). 

 ^Atkinson, G. F., Bull. Cornell Exp. Station, No. 8H, 1893. 

 ^DeVries, Bot. Zeitung, 1877, S. 1-10. 

 ^DeVries, Bot. Zeitung, 1883, S. 850. 



