HOW WATER MOVES THROUGH THE PLANT. 5 1 



u 



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



:cted with the roots, by the use of rubber tubing as shown in figure 42. A 



:ry small quantity of water may be poured in to mois- 



n the cut end of the stem. In a few minutes the water 



:gins to rise in the glass tube. In some cases it rises 



lite rapidly, so that the column of water can readily 



; seen to extend higher and higher up in the tube 



hen observed at quite short intervals. The height 



: this column of water is a measure of the force exerted 



f the roots. The pressure force of the roots may be 



easured also by determining the height to which it 



ill raise a column of mercury. 



Exercise 23. 



92. To make records of the experiment. — The pupils 

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

 hen for several days let them keep a record of the 

 :ight of the liquid in the tube, taken at several times 

 day if possible. 



93. Variation in root pressure, — In either 

 ise where the experiment is continued for 

 :veral days it is noticed that the column of water or of mercury 

 ses and falls at different times during the same day, that is, the 

 ilumn stands at varying heights; or in other words the root 

 ressure varies during the day. With some plants it has been 

 mnd that the pressure is greatest at certain times of the day, or at 

 attain seasons of the year. Such variation of root pressure ex- 

 ibits what is termed a periodicity, and in the case of some plants 

 tere is a daily periodicity; while in others there is in addition an 

 inual periodicity. With the grape vine the root pressure is 

 ■eatest in the forenoon, and decreases from 12-6 p.m., while 

 ith the sunflower it is greatest before 10 a.m. , when it begins to 

 gcrease. Temperature of the soil is one of the most important 

 eternal conditions affecting the activity of root pressure. 



Fig. 43. 

 Experiment to 

 show root pressure. 

 (Detmer.) 



II. The Loss or Water by Plants (Transpiration). 



94. Wilting of cut shoots. — We should now inquire if all the 

 ater which is taken up in excess of that which actually suffices 



