i THE NATURE OF TRANSPIRATION 5 



vacuole. A concentration of the vacuole results, and con- 

 sequently the osmotic pull on the water in the trachea 

 is increased. Hence it follows that a transference of water 

 from the tracheae will take place so long as the vapour 

 pressure of water in the tracheae is greater than that in 

 the intercellular spaces of the leaf. 



With such a mechanism before our minds we can easily 

 realise that the whole process of transpiration may be 

 purely physical and one into which vital actions (viz., 

 actions connected with living substance the intermediate 

 steps of which are little understood) need not enter. 



These considerations also explain several observations 

 otherwise difficult of interpretation. When colouring 

 materials {e.g., eosin) or other substances, poisonous or 

 non-poisonous {e.g., copper sulphate, picric acid, tartaric 

 acid, sodium bicarbonate, ferricyanide of potassium, etc.) 

 are supplied in watery solution to the cut surface of a 

 transpiring branch, it is found that the rate of transpira- 

 tion continues without much diminution till the solution 

 can be detected in the leaves, but then suddenly falls off. 

 This may be seen either by direct measurement of the water 

 drawn up, or by the fact that the leaves of these branches 

 usually remain fresh until the arrival of the solution in 

 them, but then rapidly lose their turgor, flag, and 

 finally become dry and crisp. These observations are 

 easily explained if we may assume that at first the solu- 

 tion is drawn up in the conduits under the tension set up 

 through the osmotic forces of the cells of the leaves, but 

 afterwards, when the upper portion of comparatively pure 

 water has been eliminated from the plant by transpiration, 

 the solution comes into contact with the living cells and 

 modifies or destroys their osmotic properties. The latter 

 soon lose their turgor, and the leaf droops. 



That the loss of turgescence will necessarily be attended 

 by a diminution of the amount of water evaporated from 

 the leaf will appear evident when it is considered that : 



