CHAPTER VII 



OSMOTIC PRESSURES OF LEAF-CELLS 



We have seen that the force required to move the sap 

 at the rate of the transpiration current must at least be 

 equal to the pressure produced at the base of a column of 

 water which is twice the height of the transpiring tree. 

 Is it possible to obtain any measure of the force actually 

 available to produce such a stress ? It will be of interest, 

 if this measurement can be made, to know whether the 

 force is taxed to its limits to produce the upward motion 

 of -the sap or whether there is plenty of reserve. 



A gauge for measuring this available force is provided 

 in. the leaf cells of the transpiring tree. 



Forces available for raising the sap. During 

 transpiration the cells of the leaves are normally in a 

 turgid condition. This distension is caused by the osmotic 

 pressure of the dissolved substances acting upon the 

 protoplasmic membranes of the cells and pressing them 

 against the cell walls. We have seen that secretion or 

 evaporation abstracts water from these cells, and so tends 

 to concentrate the solutions within them. This loss of 

 water can only be made good by drawing in water from 

 the adjacent tracheae, and this pull acting on the upper 

 ends of the cohering columns of sap is propagated down- 

 wards through the tree. We may then regard secretion 

 or evaporation as the force which actually exerts the 



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