THE MATERIAL OUTGO OF PLANTS 325 



Influx of water. — Transpiration has been called a function because 

 it creates a current of water through the plant, whi< h was falsely sup- 

 posed to sweep in with it the needful mineral salts. But it is impossible 

 to reconcile this conception with present ideas of osmotic movement. 

 The only condition under which more water can enter is when, by the 

 concentration of solutes in the plant, the internal pressure of the water 

 of these solutions has been reduced; and this is precisely the tendem yof 

 evaporation. If the water and plant substance were in equilibrium, 

 evaporation from aerial parts would upset this equilibrium by reducing 

 the amount of water, which would be replaced by the entrance of water 

 at any permeable region in contact with it. Hut this would by no means 

 furnish an adequate reason for the entrance of any solute which was in 

 equilibrium before evaporation took place. On the contrary, by con- 

 centration of the solution, the tendency would be in the opposite direc- 

 tion; the solutes to which the protoplasts were permeable would emigrate. 

 And the mineral salts in question, being admissible by hypothesis, 

 would do this. Transpiration, therefore, may occasion an influx of 

 water, but not of salt ; indeed it might easily cause an outgo of salts. 



Transpiration and salts. — Transpiration has been called a function, 

 also, because it was supposed to be useful in concentrating the dilute solu- 

 tions of salts brought up to the leaves. 1 That evaporation of water from 

 the leaves would tend to do this is true, of course. But the loss of water 

 is at once compensated, under favorable conditions, by the entry of more 

 water, and the solutions are again diluted. If equilibrium were assumed 

 for the moment, then the disturbance of equilibrium by evaporation 

 would determine a movement of water to readjust it, and the solution 

 would again be brought to the same concentration. Were a liter of water 

 containing a gram of cooking salt set on the fire to boil, and were pure 

 water added as fast as it boiled away, no concentration of the salt solu- 

 tion could occur. But if salt solution were added as water evaporated, 

 the concentration of the salt would be constantly increasing. This idea 

 of the concentration of dilute solutions in the leaves by evaporation in- 

 volves, therefore, the same assumption as the other " function " assigned 

 to transpiration; namely, that water carries along with it the dissolved 

 salts, as a river current sweeps along suspended mud. But this is a mere 

 assumption, and contradicts both theory and observation of osmotic 

 movement. 



1 One popular book for children even speaks of leaves as the plant's "kitchens," where 

 the thin "soups" arc boiled down. 



