1906.] On the Transpiration Current in Plants. 45 



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 

 desiccated chamber transpired nearly three times as much as the others 

 under normal conditions of maximal transpiration. In this experiment the 

 loss of water from the enclosed branch does not represent any temporary 

 desiccation 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° C. 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 desiccated leaves are nearly 1-J- times 

 as active in transpiration as those under the normal conditions. 



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 preponder- 

 ance 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, 

 then, where the branch had only 500 leaves, while the whole tree had 9,000,000 

 (1 : 18,000), it is probable that the effectiveness of the former was much greater 

 than that of the remaining leaves of the tree. Such an over-estimate in the 

 amount transpired involves, according to the method, an exaggeration in the 

 velocity of the current in the trunk. 



The control of transpiration exercised by the freedom of supply may be 

 easily observed by means of the weighing method. The amount transpired 

 will be found to fall off as the plant exhausts the water in the soil round its 

 roots, and to rise when the soil is again rendered moist. The following figures 

 illustrate this fact in the case of a small yew-tree which was exposed to 

 conditions favourable to transpiration on seven successive days. The 

 conditions were fairly uniform, as throughout the experiment the sky was 

 lightly overcast and a light east wind blew. 



