32 



PHYSIOLOG Y. 



very small quantity of water may be poured in to moisten the cut end of the 

 stem. In a few minutes the water begins to rise in the glass tube. In some 

 8 cases it rises quite rapidly, so that the column of water can 



3 readily be seen to extend higher and higher up in the tube when 



■ —S observed at quite short intervals. The height of this column 



of water is a measure of the force exerted by the roots. The 

 pressure force of the roots may be measured also by deter- 

 mining the height to which it will raise a column of mercury. 



69. In either case where the experiment is con- 

 tinued for several days it is noticed that the column 

 of water or of mercury rises and falls at different 

 times during the same day, that is, the column stands 

 at varying heights; or in other words the root 

 pressure varies during the day. With some plants 

 it has been found that the pressure is greatest at 

 certain times of the day, or at certain seasons of the 

 year. Such variation of root pressure exhibits what 

 Experiment to i s termed a periodicity, and in the case of some 



show root press- 



ure (Detmer). plants there is a daily periodicity; while in others 

 there is in addition an annual periodicity. With the grape vine 

 the root pressure is greatest in the forenoon, and decreases from 

 12-6 p.m., while with the sunflower it is greatest before io 

 a.m., when it begins to decrease. Temperature of the soil is 

 one of the most important external conditions affecting the 

 activity of root pressure. 



