210 TRANSPIRATION AND ASCENT OF SAP ch. 



Summary. In bringing to a close this description of 

 researches on Transpiration and Ascent of Sap, it seems 

 suitable to summarise br'efly the principal conclusions 

 which they have established. 



The transpiration stream is raised by secretory actions 

 taking place in the leaf cells, or by evaporation and capil- 

 larity (imbibition) at their surfaces drawing water from 

 the tracheae. The state of saturation surrounding these 

 cells determines which of these agencies is effective. 



The configuration, physical properties, and structure of 

 the wood render the conducting tracts of plants highly 

 inefficient if regarded as a system for conveying water 

 urged upwards by pressure or drawn upwards in the 

 substance of the woody walls. The distribution of living 

 cells in these tracts is such that their actions cannot 

 account for the rise of water observed, and there is no 

 reason to believe that the elimination of these activities, 

 if attended by no secondary changes in the conducting 

 tracts or transpiring leaves, will arrest the transpiration 

 stream. 



While thus structural and physiological evidence pre- 

 vent us from accepting any of the previous physical or 

 vital theories, the same Configuration, physical properties, 

 and structure of the wood compel us to admit that the 

 water in the conducting tracts, when not acted upon by 

 a vis a tergo, must pass into a state of tension. This state 

 is necessitated by the physical properties of water when 

 contained in a completely wetted, rigid and permeable 

 substance which is divided into compartments. There- 

 fore when root pressure is not acting and when the leaves 

 of trees are transpiring, the cohesion of their sap ex- 

 plains fully the transmission of the tension downwards, 

 and consequently explains the rise of the sap. 



Resistance to a current of water moving through wood 

 at the velocity of the transpiration stream is approxi- 

 mately equivalent to a head of water equal in length to 



