122 TRANSPIRATION AND ASCENT OF SAP ch. 



brisk breeze, air temperature being about 24 in the 

 month of July, the amount transpired was 5 '171 

 grammes per hour, or 0*544 gramme per 1000 leaves. 

 The amount transpired by the desiccated branch, on the 

 other hand, was 0*781 gramme per hour, or 1*502 grammes 

 per 1000 leaves per hour. Hence, the leaves in the desic- 

 cated chamber transpired nearly three times as much as 

 the others under normal conditions of maximum trans- 

 piration. In this experiment the loss of water from the 

 enclosed branch does not represent any temporary desic- 

 cation of the surface tissues, for before the weighings 

 were made the branch had been enclosed in the flask for 

 a day. If this initial desiccation had been included, as 

 it was in Ewart's experiments, the difference between the 

 amounts given off by the enclosed and unenclosed leaves 

 would have appeared greater. 



In diffuse light the difference is not so marked. The 

 unenclosed leaves of the tree used in the last experiment 

 in diffuse light with a temperature of 21 transpired 

 1*250 grammes per hour, or 0*130 gramme per 1000 leaves ; 

 the enclosed leaves simultaneously transpired 0*099 

 gramme, or 0*192 per 1000 leaves per hour. The desic- 

 cated leaves are nearly one and a half times as active in 

 transpiration as those under the normal conditions. 



Transpiration controlled by supply. These 

 experiments show that it is not justifiable to assume 

 that the rate at which water is given off by an 

 isolated branch under conditions of abnormal desiccation 

 is attained by all the branches when all alike are exposed 

 to conditions most favourable to transpiration. The 

 excess evaporation from the desiccated leaves will be 

 greater when the bulk of the isolated branch is but a small 

 fraction of the bulk of the whole tree ; for the greater 

 the preponderance of the latter the larger will be the 

 supply available for the branch, and, consequently, the 

 less the resistance to transpiration. In Ewart's experiment, 



