i THE NATURE OF TRANSPIRATION 7 



several observations seem to support the view that under 

 normal conditions some sort of vital action intervenes. 



Transpiration of living and dead leaves. It 

 was noted above that when the leaves of a branch 

 are killed, not only is the flow of water upwards greatly 

 reduced, from causes which have been already explained, 

 but ultimately the leaves dry up and finally fail entirely 

 to raise water in the branch. It is true that in this case, 

 not only are vital actions removed, but also one of the 

 most important features of the mechanism, viz., the semi- 

 permeable membrane, is destroyed by the coagulation of 

 the protoplasm. It is evident, however, that after this the 

 capillary forces of the cell- wall of the leaf- cells alone are 

 unable to continue to raise the water under the new con- 

 ditions, and this would suggest that unaided they may be 

 insufficient in the living leaf. 



This line of reasoning would indicate that the protoplasm 

 may not only act as a semi-permeable membrane by allow- 

 ing water to pass through to a region of diminished pressure, 

 but it may also actively secrete water, or a solution, on 

 its outer surface. Such glandular action of protoplasm is 

 well known, and examples from the coenocytic fungi may 

 serve as illustration. In Phycomyces, Pilobolus, and 

 Mucor, water which is absorbed by the submerged part of 

 the coenocyte is expelled on its aerial surface. The same 

 process may be witnessed in the nectaries and all the 

 glands of higher plants. 



In these cases there is some reason to believe that the 

 liquid exuded is not pure water ; and hence it seems pro- 

 bable that the protoplasm first secretes soluble substances 

 on its outer surface and that these act osmotically when 

 concentrated by evaporation, on the vacuoles within and 

 draw water from the cells. Where the process is very 

 rapid, as, for example, the expulsion of water from the 

 cells of the pulvinus of Mimosa in response to a stimulus 

 such a sequence of events can scarcely be imagined. The 



