i THE NATURE OF TRANSPIRATION 9 



energy is being expended, and the energy apparently must 

 be supplied by the metabolism of the protoplasm. 



Fortunately the quantities exuded from the leaf tips of 

 Colocasia are quite large enough to allow us carefully to 

 test the purity of the water. With this intent some 10 c.c. 

 were collected from the leaves of this plant during one 

 night ; and, by means of the thermo-electric method of 

 cryoscopy, which will be described later, were tested with 

 regard to their freezing-point. It was found not to differ 

 sensibly from that of distilled water. The electrical con- 

 ductivity was also determined for the sample, and it was 

 found to be less than that of tap water. These tests show 

 that osmotically we may regard the exudation as pure 

 water, and, consequently, the process must be one of secre- 

 tion involving the intervention of living protoplasm and 

 the expenditure of stored energy. 



In the case of Colocasia the dripping ceases when the 

 leaves are surrounded with an unsaturated atmosphere. 

 Evidently the secretion is no more than able to keep pace 

 with the demand of evaporation, but at the same time it 

 seems reasonable to assume that the water has been largely 

 supplied for evaporation by a secretory process. It seems 

 also reasonable to suppose that when evaporation is acceler- 

 ated beyond the capacities of protoplasm for secretion 

 the cell-walls will dry and the capillary forces of the 

 menisci in their substance will lend their aid in separating 

 the solvent from the solutions in the vacuoles. 



How far can we transfer the glandular functions of the 

 leaf tip of Colocasia to the mesophyll cells of the trans- 

 piring leaves ? An answer to this question has been 

 sought by different ways. 



Transpiration into various gases. If the cells 

 of the mesophyll secrete pure water on their outer side 

 we should be justified in expecting that this secretion, 

 in common with other vital actions, would be inhibited 

 when the protoplasm was anaesthetised, or would be 



