34 PHYSIOLOGY. 



plants in the open may be used if it is preferred, but plants grown In pots 



are also very serviceable, and one may use a potted begonia or balsam, the 



latter being especially useful. The plants are usually convenient to obtain 



. from the greenhouses, to illustrate this phenomenon. 



jl I The stem is cut off rather close to the soil and a long 



A glass tube is attached to the cut end of the stem, still 



connected with the roots, by the use of rubber tubing, 



as shown in figure 45, and a very small quantity of v,-ater 



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 cases it rises quite rapidly, so that the 



column of water can readily be seen to extend higher 



1 and higher up in the tube when observed at quite 



1 short intervals. (To measure the force of root pressure 



is rather difficult for elementary work. To measure it 



tf^EBlfc see Ganong, Plant Physiology, pp. 67, 68, or some other 



Sf book for advanced work.) 



_JH ; v 67. In either case where the experiment is 



MB fj& continued for several days it is noticed that the 



Pig. 4S . column of water or of mercury rises and falls at 



sh^w^cSr^ressure different times during the same day, that is, the 

 column stands at varying heights; or in other 

 words the root presssure 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 is termed a periodicity, and in 

 the case of some 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 10 A.M., when it begins to decrease. Temperature of 

 the soil is one of the most important external conditions affect- 

 ing the activity of root pressure. 



