13 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



The relative transpiration (frequently called also the "transpiration 

 capacity'' or "transpiring power") is a very valuable concept in 

 botanical science, since it enables one to form an idea of the combined 

 effect of the factors which influence the transpiration rate as modified 

 both by external conditions and by the internal anatomical structure 

 of the plant. For instance, if the mean transpiration capacity, during 

 the day, of Betula is 0.59, of Quercus 0.16, and of Pinus 0.06, these 

 figures serve to indicate the relative protection against water loss 

 possessed by these three plants, as pointed out by Huber {106). 



The subject of transpiration has been studied more than any other 

 phase of water relations. Most of the experiments, to be sure, have 

 been carried on with herbaceous plants, but there is little reason to 

 doubt that the principles of transpiration hold throughout the plant 

 kingdom, whether one is dealing with herbaceous or woody forms. In 

 the following pages the subject will be reviewed very briefly with 

 special reference to the literature on trees, in order that the reader 

 may form a fairly adequate concept of the work that has been done 

 in this field and of the organization of the subject ; but for more detailed 

 summaries he is referred to Burgerstein (28) and the more recent 

 work of Seybold {189). A part of this literature has been reviewed 

 also by Maximov {148), but he has been interested primarily in soil- 

 moisture studies with special reference to plants of dry climates, and 

 for this reason this last-named work is not so comprehensive as the 

 monographs of Burgerstein and Seybold. It might also be mentioned 

 that Seybold has not duplicated the field covered by Burgerstein, who 

 devoted himself largely to the older and more classical phases of 

 transpiration, in that he studied largely the factors influencing trans- 

 piration and the methods by which plants check transpiration. Sey- 

 bold, on the other hand, has considered the question more from the 

 point of view of the recent developments in physics and chemistry. 

 The two monographs, therefore, supplement rather than duplicate 

 each other. TVhile both Burgerstein and Maximov devoted some 

 attention to the ecological aspects of the question, these phases are 

 developed much more fully by Seybold. 



METHODS OF STUDYING TRANSPIRATION 



The methods of studying transpiration may be outlined as follows: 



I. Determination of amount of water absorbed by the plant to compensate 

 for that lost by transpiration — potometers and lysimeters. 

 II. Measurement of stomatal openings. 



III. Collection or determination of the water vapor transpired. 



a. Collecting the moisture given off with a hvgroscopic substance, such 



as P 2 5 or H 2 S0 4 . 



b. Measuring the change in humidity of the surrounding atmosphere with 



a hygrometer. 



c. Using some sort of "indicator"' applied to the leaf. 



1. Cobalt chloride paper. 



2, Collodion. 



IV. Determination of changes in weight of the plant due to loss of water. 



a. Using cut parts. 



b. Using entire parts. 



Methods I and II are both indirect, inasmuch as they do not consider 

 the actual water losses. Method I — the determination of the water 

 absorbed by the plant — can be only a rough approximation^ since all 

 of the water absorbed is not lost; part is utilized for the various pur- 

 poses to which water is put and which were discussed at the beginning 



