Vol. 32 



No. 10 



BULLETIN 



OF THE 



TORREY BOTANICAL CLUB 



OCTOBER, 1905 



A new method of measuring the transpiration of plants in place* 



William Austin Cannon 



Very early in the history of the Desert Botanical Laboratory, 

 it was recognized that the study of the transpiration of desert plants 

 ought to be one of the leading lines of research. It was appreci- 

 ated also that to be most effective the study should be confined as 

 far as possible to plants growling under natural conditions, that is, 

 to plants in the field. 



That the task is not without its difficulties is readily seen. The 

 forms of apparatus in general use are not intended and in fact are 

 not well adapted for the determination of the transpiration of plants 

 in place. It was necessary therefore in the very beginning to revise 

 old methods, or to originate and to perfect a new one, both of 

 which have been done.f 



It is not to be understood that the method to be described, 

 which was devised to meet the special need, is either ideal or has 

 reached its final stage of development But the use of it for a 

 year in many experiments out-of-doors and under different condi- 

 tions has shown it to be so practicable that a description of it at 

 this time, together with some reference to experiments performed 

 with it, appears to be desirable. 



The polymeter method 



Several methods are in general use by which the amount of 

 watery vapor given off by plants may either be estimated, or 



Papers from the Desert Botanical Laboratory of the Carnegie Institution, No. 5. 



t This paper is devoted to the exposition of the method most used in 1904. Others 

 are now being tried ; should any of these appear to be satisfactory they will be described 

 at another time. 



[The Bulletin for September (32 : 451-514) was issued 21 S 1905.] 



515 



