Grafts etal. —184— Water in Plants 



literature on transpiration it is obvious that only a small portion can be 

 treated here. 



Methods of Measuring Transpiration: — The methods which have 

 been extensively utilized in determining the rate of water loss by a transpir- 

 ing plant logically fall into three groups: determinations of 1) the loss of 

 weight by whole plants or plant parts, 2) the amount of water vapor given 

 off, and 3) the rate of water absorption. Detailed considerations of the 

 various techniques employed with descriptions of the types of apparatus 

 used and the numerous modifications suggested are given by Burgerstein 

 (1920), Grafe (1924), Maximov (1929a), and Leick (1939). 



While it has the advantage that the transpiring organs need not be en- 

 closed in an artificial atmosphere which would modify the rate of water 

 loss, the unwieldiness of the loss in weight method has somewhat restricted 

 its use particularly when ecological experiments with numerous large plants 

 have been desired. 



If the surfaces of the culture vessel in which a plant is growing are 

 sealed in a way such that water loss is restricted to the plant itself, changes 

 in weight are brought about by transpiration (lenticular, cuticular, and 

 stomatal) combined with the processes of metabolism (catabolic and ana- 

 bolic). Under normal conditions of even moderate or reduced transpira- 

 tion, the loss of water vapor completely overshadows any changes due to 

 other physiological processes in the plant. Periodic weighings coupled with 

 accurate determinations of leaf area, or green or dry weight enables the 

 investigator to express the rate of transpiration per unit of plant area or 

 weight. 



Several problems arise, however, which make the analysis difficult. 

 These are chiefly concerned with abnormalities in response of culture 

 plants compared with those growing undisturbed in the field. Obviously, 

 injury to the root system of the experimental plant through transplanting 

 to a culture vessel immediately before making observations on transpira- 

 tion rate is to be avoided. Not so apparent, however, are the modifications 

 which may arise in plants grown continually in pots or large containers. 

 Control "of nutrition, water supply, soil pU, and air supply to the roots are 

 among the most troublesome problems. These may be appreciated when 

 one realizes the volume of soil which the roots of single herbaceous plants 

 may permeate under natural conditions. Not only may the water supply 

 be abnormal in potted plants but stomatal response (because of variation in 

 water supply or due to some other cause) may be different in potted as com- 

 pared with field grown plants. Obviously the transpiration rate from such 

 an experimental plant would be far from comparable with that of its field 

 grown complement. 



Nevertheless, the loss in weight method when employed critically has 

 been one of the most useful, particularly for laboratory and greenhouse 

 experiments, as well as for certain ecological and irrigation studies such as 

 those carried out by Briggs and Shantz (1913, 1916), Kiesselbach 

 (1916), Veihmeyer (1927), and Veihmeyer and Hendrickson (1927). 

 Various automatic recording devices have been developed to obtain con- 

 tinuous records of weight loss (Briggs and Shantz, 1915; Brauner, 

 1931 ; ScHRATZ, 1931 ; and others). 



The unwieldiness of using whole plants in the weight method has been 

 circumvented by modifications whereby plant parts are removed and their 

 loss in weight while standing in water recorded. Such a drastic treatment 



