422 . BOTANICAL GAZETTE [December 



The gradual decline of the transpiring power of broad-leaved 

 trees preceding defoliation and during the formation of the absciss- 

 layer is what might be expected, but is heretofore, we believe, 

 unrecorded. Weather conditions play an important part in 

 hastening or retarding this process, and as a result the dropping off 

 in the transpiration graph. These graphs are surprisingly similar 

 in their general trend to those of conifers where the leaves are 

 intact. 



The spring of 1918 was unfavorable for rapid leafing out or 

 renewed growth of trees. Consequently the increased transpiring 

 power, which is in a direct ratio with the leaf area, occurred very 

 gradually, and here again was not greatly unlike that of the needle- 

 leaved trees. Undoubtedly under very favorable weather condi- 

 tions, as occur during some springs, the transpiring power might 

 easily double or treble day by day for a rather long consecutive 

 period. 



Summary 



1. Autumn transpiration losses from conifers are just as great 

 as or even greater than those from broad-leaves. 



2. The decrease in water losses from broad-leaved trees resulting 

 from defoliation is gradual, and not greatly unlike the decrease 

 shown in the transpiring power of conifers. 



3. Winter transpiration losses from conifers are only 1/55- 

 1/2 5 1 as great as those in autumn. 



4. The increased losses of broad-leaved trees in spring occa- 

 sioned by foliation are in proportion to the leaf areas exposed, and 

 are closely controlled by weather conditions, but in the main are 

 similar to increased losses of conifers. 



5. Winter transpiration losses from conifers are scarcely greater 

 than those from defoliated stems of broad-leaved trees. 



The writers are indebted to Dr. R. J. Pool for his kindly 

 interest and encouragement, and to Dr. L. J. Briggs for wilting 

 coefficient determinations; also to Mr. J.. Higgins, of Halsey, 

 Nebraska, and Mr. T. J. Larsen, Priest River, Idaho, for tree 



