528 Cannon: A method of measuring transpiration 



1. The polymeter method of determining the transpiration of 

 plants IS based upon changes in the relative humidity of the at- 

 mosphere in which the plants are placed. From data respecting 

 the relative humidity and the temperature, provided by the poly- 

 meter, the absolute humidity is calculated by well-known meth- 

 ods. The difference in the absolute humidity constitutes the 

 amount transpired. 



2. The results derived from the use of the method are shown 

 by laboratory tests to be consistent, and, provided certain precau- 

 tions are taken, to be reasonably accurate. 



3. The apparatus is easily adjusted, it is not likely to get out 

 of order, and it may be easily transported. 



4. The new method is especially suitable for use in a dry 

 locality ; where the humidity is high some other method will 

 probably be more satisfactory. 



. 5. That the range of the use of the method outdoors is large 

 will be perceived by the following list which is a part only of the 

 plants studied : Cerens giganteiis^ CovilUa tridentata^ Encelia 

 farinosa^ Fonqideria splendens^ Fraxiyms arizonica, Phoradendron 

 macrophylliun, Pinus ponderosa^ Qiiercus oblongifolia, Kallstroemia 

 maxima^ and Verbena ciliata. 



6. It will be observed by the list that not only entire plants 

 in place, but also portions of plants while still attached, can be 

 employed. This greatly enhances the usefulness of the method, 

 as the following indicates : In a certain experiment on a mistletoe 

 and its host, the transpiration of the mistletoe was taken while it 

 was still growing on its host and then the apparatus was read- 

 justed and the rate of the host branch upon which the mistletoe 

 grew was observed. When these experiments were completed 

 the transpiration of a seedling of the same species as the host 

 plant was learned. Thus results of much pathological and 

 physiological importance w^r^ surely and readily obtained. 



7. It is important to note that a plant can be experimented 

 upon repeatedly without apparent injury, which makes it possible 

 to observe the transpiration at hourly or at daily intervals, and 

 thus to learn the diurnal or the seasonal variations as desirable. 

 As a single instance of seasonal differences in rate, the transpira- 

 tion of Fouqiikna may be cited. The absolute seasonal variation 



