iqiq] weaver &• MOGENSEN— TRANSPIRATION 421 



Conclusions 



A consideration of the foregoing data leads us to some con- 

 clusions quite the converse of statements generally current in 

 ecological-physiological literature. Perhaps the most important of 

 these are the facts shown to hold under the conditions of these 

 experiments; first, that broad-leaved trees imder late summer 

 conditions have no greater and indeed often a smaller transpiring 

 power, area for area, than conifers; and secondly, that the water 

 losses of coniferous trees during the winter months are relatively 

 no greater with the needles intact than are the losses from deciduous 

 trees after the leaves have fallen. 



The fact that all of the plants concerned, including three species 

 and about 30 individuals of broad-leaved trees, and six species 

 represented by about 70 individuals of conifers, gave results which 

 without exception point to these conclusions, leaves little doubt 

 as to the validity of the finds. These results were obtained with 

 all the plants under uniform conditions of soil type and texture, 

 soil temperature, and identical aerial environment. The only 

 variable factor was soil moisture, and here the range was no greater 

 for the one group than for the other. Although the experiments 

 were undertaken near the end of the summer, for a period of a 

 few weeks, after their beginning the deciduous trees were in excellent 

 growing condition, and the comparative losses as here recorded 

 occurred during the earlier part of this period, when the weather 

 was similar to that of midsummer as regards temperature and 

 humidity. 



That winter losses from the same leaves that transpired so 

 freely the preceding fall and again in the following spring are so 

 small is certainly testimony of the ecological efficiency of coniferous 

 leaf structure for reducing water losses. Whether this is due 

 entirely to stomatal closure, or, as seems more probable, is con- 

 nected with chemical changes in cell contents as well, remains to 

 be determined. Such work as that of Miyake (15) on the food 

 making of coniferous leaves in winter and Ehlers (5) on tempera- 

 ture is rapidly throwing considerable light upon the winter activ- 

 ities of coniferous trees. 



