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, which 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 tendency of 
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. But 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" are boiled down. 
