522. Cannon : A method of measuring transpiration 



base IS also frequently more convenient when studying the trans- 

 piration of such a plant as the mistletoe (figure 5), or a branch of 

 a tree. Wliatever may be the style of base used, great care must 

 be taken to avoid any leakage which would render the results 

 quite untrustworthy. 



A very necessary addition to the apparatus as above presented, 

 in this country of high temperatures and bright sunlight, Is some 

 form of shade. Several types have been used. The one given in 

 FIGURE I is perhaps the most satisfactory. It is made of heavy 

 white duck. Straps are attached so that it can be fastened to 

 camera tripods ; it is thus easily carried and is very rigid w^hen 

 spread, and flaps are sewed to the bottom, upon which stones are 

 piled to ensure a safe anchorage against the winds. 



The manner of setting up the apparatus is very simple. As 

 soon as the base \s arranged, the polymeter is placed upon it in a 

 position where it may be readily observed, and the bell-glass is 

 placed over the plant and the polymeter and sealed quickly and 

 tightly to the base. The entire apparatus is shielded from the 

 direct sunlight, which can be done just before or immediately 

 after the bell is put in position. The adjustment of the apparatus 

 need not take longer than three or four minutes. 



When the apparatus is set up, the time, the relative humidity 

 and the temperature are at once taken and recorded. The read- 

 ings should be repeated at frequent and regular intervals. 



The calculation of the absolute humidity 



The amount of moisture in the atmosphere of the bell-glass 

 may be found by a simple calculation, which may be based wholly, 

 or if preferred only in part, on data provided by the polymeter. 

 The manner of reducing the observations in both instances may 

 be illustrated by the data given in the following experiment : 



Time Relative humidity Temperature 



9:33 A. M. 23 29 ° C (84.2^ F.) 



10:00 '* 50 The same 



The bell has a capacity of 17,500 c,c. 



A method of deriving the absolute humidity with the aid of 

 data given entirely by the polymeter will be given first. 



