Transpiration and the Ascent of Sap. 5 



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 experiences 

 in passing through the cell-walls and protoplasm. In the present 

 instance this force is the difference of vapour 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 tlien 

 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 occupied 

 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 extremely high 

 value owing to the fineness of the texture of the cellulose. The con- 

 traction of cellulose on drying, involving the reduction of these 

 passages, enhances this effect so that the capillary traction 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 consequently 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 flow across the cell is determined by the difference 

 of vapour pressure on its opposite sides. 



As the pressure conditions obtaining in the transpiring organs 

 at and near the evaporating surfaces have given rise to some 

 obscurity, we may devote a little space to their closer consideration. 

 In the simplest case a cell evaporating into an intercellular space 

 abuts directly at its inner side on a trachea filled with water. The 

 conditions may be illustrated by the model figured (Fig. 2). It con- 



