172 BOTANICAL GAZETTE [September 



times as much energy as assimilation, which in turn uses eighty 

 times as much as it takes to lift the water ; obviously this 

 depends upon how far this is lifted. 



It must be almost exclusively true in nature, as Askenasy 

 (^^95 : 333) has it in his very clear and concise scheme of the 

 energy changes in the rise of water, that the chief force in the 

 leaf which removes the sap from the tracheae and in spite of 

 which it evaporates, is osmosis. To be complete, it may be said 

 that imbibition intervenes between osmosis and evaooration to 

 get the sap through the wall of the mesophyll cells from which 

 most of it evaporates. On the other side, it also intervenes in 

 the walls between the living contents of the mesophyll and the 

 tracheae from which they draw water. Very likely a little water 

 passes out in the walls from the vessels to the point of evapora- 

 tion independently of osmosis; at any rate this is not prevented 

 by suberization of the anticlinal walls, as in the endodermis of 

 roots. When water evaporates from a mesophyll cell, the con- 

 centration of its sap increases and the tension of its wall is elim- 

 inated. In both of these ways, but at first in chief part in the 

 second way, the cell is enabled to draw more water from whence 

 it can, from the tracheae. Mechanically this elimination of the 

 tension of the wall suggests the elasticity of the wall by which 

 Bohm (1S64) at first sought to explain suction by leaves; but 

 the actual tension alwavs remains a stretchinof. In some of his 

 latest work (1890, etc.) the same author denied the play of 

 osmosis on the ground that transpiration is not stopped by 

 death. Evaporation does occur after death, evaporation of the 

 water in the leaves, and of as much more water as can be drawn 

 up before enough air diffuses into the walls and lumina of the 

 tracheae to close them to water; \n leaves of firm texture, espe- 

 cially in evergreens, this does not soon occur. But this is not 

 ground for doubting that in living leaves the water passes 

 through the cells, necessarily, by osmosis. 



All water passes from the tracheae by imbibition, and so far- 

 as the mechanism of movement in the tracheae is concerned, it 

 matters not whether or not osmosis is active in the leaves (Ask- 



nJl 



