394 BOTANICAL GAZETTE [December 



factors of the habitat under which the plants are growing except 

 as measured by the internal transpiring power of the plant, nor 

 does it give a record of the continuous transpiration losses. 



Unquestionably the most reliable data have been those obtained 

 by the potometer, where the entire sealed container was weighed 

 with the whole plant intact. A survey of the literature, however, 

 reveals rather scant data on the transpiring power of trees, especially 

 coniferous trees, regardless of the method employed. 



That evergreen trees are constantly supplied with water, even 

 in winter, was first observed by Hales (8), and later by Duhamel 

 (4), Treviranus (18), and others. 



In i860 Hartig (10) made some investigations on transpiration 

 losses with Picea, a meter high, in milder winter, and found that 

 the plant lost from about 100-125 gm. of water a day. These 

 figures, however, are of little value so far as calculating the intensity 

 of transpiration is concerned, since he gave neither area nor weight 

 of the transpiring part. 



BuRGERSTEiN (3) in 1875 indicated the relation of transpiration 

 to lower temperatures, and showed that cut branches of Taxus 

 baccata transpired in an hour, at —2° C, 0.288 per cent, and at 

 — 10.7° C, 0.019 per cent of their fresh weight. 



That transpiration may take place quite rapidly at rather low 

 temperatures has been shown by Wiesner and Pacher (22). 

 Twigs of Aesculus and Quercus, for example, lost 0.32 and 0.25 

 per cent respectively of their weight in 24 hours at —3.5° to 

 — 10.5° C, and 0.199 and 0.192 per cent at —5.5° C. to —13.0° C. 



Beach and Allen (i) found a loss of from 4 to 9 per cent of 

 water in apple twigs during a single week in January, with a 

 minimum temperature of —26.0° C. They found also in general 

 that the hardiest varieties were the most resistant to water loss. 



According to Warming (20, p. 310), coniferous trees exhale 

 much less water vapor than dicotyledonous trees, due to their 

 xerophytic nature. 



Kusano (12) has given convincing quantitative data on the 

 transpiration of evergreen trees indigenous to Japan. He found 

 that evergreen trees transpired in winter an average quantity of 

 at least o .48 gm. per sq. dm. per day (with the exception of conifers). 



