THE LIFE OF A PLANT 



69 



the top of a plant. This can be proved differently by getting 

 a short glass tube, and a plant, say a geranium, whose stem 

 is about the same size. Cut the plant off near the ground, 

 and bind the tube to the stem with a strip of plaster. Sap, 

 forced out by the pressure from 

 the roots, will gather in the tube. 



But why should the soil water 

 pass upward? To understand 

 this, let us think once more 

 about denser and lighter solu- 

 tions. We have seen that we 

 can make a solution denser and 

 denser by adding salt. But of 

 course the same result can be 

 obtained, from the same solu- 

 tion, by taking away water. If 

 we simmer it on the stove, the 

 water will pass off (we call it 

 evaporating) and the solution 

 will taste saltier and saltier. 



Now take any plant which can 

 be covered by a tumbler. Put 

 cardboard over its pot, and seal 

 the slit in the cardboard, as 

 before. Then turn the tumbler 

 upside down over the plant, standing it on the cardboard. 

 In a couple of hours water will gather on the glass. It 

 must have come from the leaves of the plant, from which it 

 evaporated. The amount of this evaporation can be 

 roughly measured. See Fig. 40. 



It is now plain that the leaves of a plant are continually 

 evaporating a part of the water that comes from the roots. 

 But if this is always being evaporated, then the liquid 



FIG. 41. A means of measur- 

 ing the amount of water evap- 

 orated by a plant's leaves, when 

 watered through the thistle-tube. 



