410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



water from the cell so as to diminish its total water content, quite 

 a small difference of pressure will cause water to move across the cell 

 when it is distended to its maximum with water. The osmotic pres- 

 sure of the cell then acts simply as a force pushing the protoplasmic 

 lining against its walls, while the water on one side of the cell is free 

 to move across to the other side except for the resistance it expe- 

 riences in passing through the cell walls and protoplasm. In the 

 present instance this force is the difference of vapor pressure existing 

 on the inner and the outer, or evaporating, side of the mesophyll cell. 

 It might happen that this difference would be sufficient to almost 

 keep the wall on the evaporating side flooded with water, and then 

 evaporation into the intercellular space would take place as if from 

 a free liquid surface; or, if evaporation proceeded more rapidly, the 

 liquid surface might retreat into the substance of the evaporating 

 wall. Then the capillary or imbibitional properties of the wall would 

 exert a force drawing the water through the cell and bringing it to 

 the surface of evaporation. The retreat of the water surface would 

 proceed till the capillary forces so produced could bring forward 

 water as fast as it evaporated from the surface and a steady state 

 were arrived at. 



According to this point of view the submicroscopic spaces occu- 

 pied by the imbibed water in the cell walls are regarded as intensely 

 minute capillary passages. When water is lost, the surface of that 

 which remains behind retreats in the form of innumerable menisci 

 into these spaces. The capillary forces intensify as these menisci 

 increase the sharpness of their curvature, and may attain an ex- 

 tremely high value owing to the fineness of the texture of the cellu- 

 lose. The contraction of cellulose on drying, involving the reduc- 

 tion of these passages, enhances this effect so that the capillary trac- 

 tion drawing the water from the cell within may become enormous. 



If the supply coming into the cells were small compared with the 

 evaporation, it might be that the steady state would not be attained 

 until the capillary forces, bringing water forward as it evaporated, 

 had actually reduced the volume of water in the cell and conse- 

 quently reduced its turgor. Under these conditions we would have 

 the capillary forces of the outer cell wall pitted against the osmotic 

 solutions in the cell itself, and, if exerting a superior force, drawing 

 water into and across the cell, now somewhat diminished in size and 

 containing a more concentrated solution; but, all the same, the How 

 across the cell is determined by the difference of vapor pressure on 

 its opposite sides. * * * 



The most vexed problem of the ascent of sap is how the water 

 rises in the stem to fill the tracheae of the leaves. 



Botanists have sought solutions of this problem in two directions, 

 viz: (1) In the energy transformations taking place in the living 



