47° BOTANICAL GAZETTE [JUNE 
of this xerophily, this necessity of keeping the transpiration within 
the limits of absorption and conduction. 
From the experimental work it was evident that, supplementing 
any water vapor absorbed by the leaves, the rates of absorption 
and conduction in the evergreen ericads are more than ample to 
furnish sufficient water for transpiration during warm spells in 
winter and for continued periods of cold temperature down to at 
least —15°. When, however, continued temperatures lower than 
— 20° prevail in this region, the rate of conduction is not sufficient 
to prevent drying and death of the exposed parts of the plant. On 
this point the winter of 1911-1912 gave remarkable evidence. 
Consequently, a high degree of xeromorphism is demanded to 
compensate for a lack of a decrease of leaf surface during the winter. 
EXPERIMENTATION DURING THE SUMMER 
Trans piration 
The water loss of plants is greatest in amount during the sum- 
mer or growing season. The greater evaporating power of the 
air, the greater exposure of surface of most plants, the greater 
availability of water, and the use of water in photosynthesis all 
express the direct opposite of the conditions in winter and show 
the greater need for water in the summer. The results of a few 
typical experiments have been plotted in figs. 7-9. 
During the first week of July 1912, the transpiration of three 
series of potted plants was determined at frequent intervals (fig. 10). 
As this week was one of extremely hot, dry weather, it will serve 
to indicate the limits of transpiration to which these plants may 
be subjected insummer. At the beginning of each series of potted 
plants a number of cuttings were also run to enable a comparison 
between cuttings and potted plants of the same species. 
With the potted plants it is noteworthy that, with but a single 
exception, the period of maximum transpiration was reached at the 
time of maximum evaporating power of the air, a little before the 
middle of the afternoon. The graphs of evaporating power of the 
air and those of transpiration of potted plants are very similar. 
Per unit area, however, between three and five times as much 
