HOW WATER MOVES THROUGH THE PLANT. 5 1 



the soil and attach a long glass tube to the cut end of the stem, still con- 

 nected with the roots, by the use of rubber tubing as shewn in figure 42. A 

 very small quantity of water may be poured in to mois- 

 ten 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 and higher up in the tube 

 when observed at quite short intervals. The height 

 cf 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 determining the height to which it 

 will raise a column of mercury. 



Exercise 23. 



92. To make records of the experiment. The pupils 

 can take notes on the experiment at the time it is set up. 

 Then for several days let them keep a record of the 

 height of the liquid in the tube, taken at several times 

 a day if possible. 



93. Variation in root pressure. In either 

 case where the experiment is continued 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 ex- 

 hibits 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 soilis one of the most important 

 external conditions affecting the activity of root pressure. 



II. THE Loss OF WATER EY PLANTS (TRANSPIRATION). 



94. Wilting of cut shoots. W r e should now inquire if all the 

 water which is taken up in excess cf that which actually suffices 



Fig. 4. 



Experiment to 

 show root pressure. 

 (Detmer.) 



