64 



PLANT PHYSIOLOGY AND ECOLOGY 



The effects of slope, exposure, and cover upon the amount of water 

 loss are exerted through the factors already discussed. 



8 1. The measurement of transpiration. Of the many methods 

 used to determine the amount of water lost from a transpiring 

 plant, the most accurate and satisfactory is to weigh the plant 

 in its own soil from time to time. This can easily be done in 

 the case of plants grown in pots. A vigorous plant, growing 

 preferably in a pot 1 or IJ dcm. in size, should be chosen for 

 study. The entire pot is covered with sheet rubber to prevent 



Fig. 17. Treatment of plants for measuring transpiration. Three are 

 rooted plants in pots; the other two are potometers, the one with a 

 rooted plant, the other with the root removed. 



the loss of moisture from its surface; or the outside of the pot 

 may be waxed, and the top alone covered with rubber. The latter 

 is tied closely about the stem, alongside of which a funnel tube 

 is placed for the purpose of supplying water and air to the soil. 

 The entire apparatus is weighed, and is then weighed again at 

 the desired intervals. The plant should be kept under study 

 for a week or two, being placed now and then in different condi- 

 tions. Weighings are made at such times as to give the water 

 loss for day and night, for periods of different length, and the 



