CHAPTER V 



TRANSPIRATION AND WATER MOVEMENT 



THE water-content of a plant is no index of the amount 

 which has been absorbed throughout its life by the root- 

 system. It is a thoroughly familiar fact that water is 

 commonly eliminated from the plant as water- vapor. 

 This elimination, termed transpiration, is important and 

 should receive special consideration. A very large pro- 

 portion of the water absorbed by plants is transpired; 

 that is, it passes into the atmosphere by diffusion through 

 the leaves and other delicate parts. This loss of water 

 may be very simply demonstrated by placing a potted plant 

 under a bell glass, taking the precaution to place a rubber 

 cloth over the pot and over all possible evaporating sur- 

 faces except the plant itself. In a short time a mistiness 

 upon the glass will indicate roughly the loss of water. 



50. Observations upon transpiration. - The demon- 

 stration of water-loss may be made in a variety of ways, 

 best of all by loss of weight. Nevertheless, single leaves 

 and abscised branches or organs may be employed in 

 various potometers, by means of which there is measured 

 the water absorbed, this latter corresponding in the end, 

 of course, very closely to that which is given off. Interest- 

 ing experiments may be readily set up with single leaves 

 or shoots (Fig. 23). By another type of experiment 



84 



