• 145 



observed bv Sachs, and is, no doubt, due to the fact that where active transuira- 

 . . . ' \ 



tion IS going on no root pressure exists, but transpiration or other current do not 



permit tlie duets to become tilled with water, but, rather, they contain rarified air 



that allows the water poured in on top of the cut surface to be drawn down in 



the stem. 



A study of the records warrant the following general statements regarding 

 the relation between temperature and root pressure : Under usual conditions 

 there can be no relation between the periodicity of root pressure and the daily 

 variations in temperature, the latter being between 50° F. and 90° F., as determ- 

 ined in the course of the experiments. Even where the periods of maximum and 

 minimum temperature were reversed in the test, and the reversed condition con- 

 tinued for several successive days, no appreciable efi'ect was noticed in the period- 

 icity of root pressure. 



The changes in temperature above or below certain limits may alter the 

 regularity of the times of maximum and minimum pressure periodicity, but do 

 not interfere with the main cycles of greater or less pressure. 



The time element is the all important one, and for most plants the period of 

 maximum pressure is 12 m., with the limits between 9 a. m. and 1 p. m. 



No appreciable difference exists between the times of maximum pressure in 

 the variety of plants studied and certainly none whatever in a large number of 

 si^ecimens of the same species even though they may have been grown under 

 different conditions. 



The age of the plant seems to make no difference in the times of maximum 

 and minimum periodicity or its general behavior in the experiment, except, as 

 would be expected, a large and vigorous plant .shows more difference between the 

 amount of maximum and minimum pressure than a small and less vigorous one. 



In different genera marked differences exist as to the maximum amount of 

 root pressure and in some it is so small that at no time can it be measured except 

 with great difiiculty. 



The amount of water present in the soil within certain limits does not affect 

 the time of periodicity or amount discharged, but in very dry soil, where the roots 

 become wilted, changes are evident as the result of the loss of the turgidity of the 

 root. 



A consideration of the relation between root pressure and the other phe- 

 nomena in living plants will be interesting in this connection. With regard to 

 its relation to transpiration, the latter can not be explained by the former, since, 

 at the most, it is not sufficient to lift the water above 80-90 feet. . Root pressure 

 furnishes only a part of the water used in transpiration, as was shown by our own 



