516 Cannon: A method of measuring transpiration 



4 



demonstrated. These methods may be grouped into three classes. 

 In the first of these the plant is potted, and weighed at intervals ; 

 the loss of weight is assumed to be due to the evaporation of water 

 from its surface. In a second class the plant is enclosed in a tight 

 receptacle and the vapor evolved from it is collected in some kind 

 of drying material which is weighed before and after the experi- 

 ment ; any increase in weight is attributed to the absorption of 

 water which was derived from the plant. In the third class the 

 amount of water transpired is estimated from the amount absorbed 

 by the plant. The apparatus employed in this class is some form 

 of potometer. 



. The mere demonstration of the fact of transpiration has been 

 made by placing the plant, or a portion of it, in a tight receptacle 

 which encloses at the same time some hygroscopic material. The 

 increase in the amount of vapor in the receptacle, or the presence 



r 



of any considerable amount, is indicated by some visible change, 

 as in the color or position, taking place in the substance.* 



The method most used the past season at the Desert Botanical 

 Laboratory is in principle a departure from any given above. It 



+ 



consists essentially in the determination of changes in the absolute 

 humidity of the atmosphere in which the plant is placed ; the dif- 

 ference in the absolute humidity, expressed in fractions of a gram, 

 is the amount of vapor transpired. The calculations by which the 

 absolute humidity is derived are based on variations of the relative 

 humidity. 



r 



An account of the apparatus 



The form of apparatus with which, as was mentioned above, 



^ 



the most of the transpiration studies in the field were carried on 

 during the past season at the Desert Botanical Laboratory was 

 suggested early in 1904 by Dr. D. T. MacDougal of the Advisory 

 Board of the laboratory. This is the apparatus used in the poly- 

 meter method for the determination of transpiration (see figure i). 

 The apparatus consists of a hygrometer, a bell-glass and a 

 water-proof base. The form of hygrometer used was Lambrecht's 

 Rtise Polymeter. This instrument is especially constructed to 

 record a variety of meteorological conditions ; it is, however, well 



*For a more detailed account of the methods used in studying the transpiration of 

 plants, see BurgeRSTEIN, Die Transpiration der Pflanzen, 1904. 



