THE FOOD OF THE PLANT 31 



reason for the absorption of such an excess of water 

 must now be looked for. 



It has aheady been seen that roots can absorb dilute 

 solutions only, that the rate of absorption is checked 

 and finally stopped altogether by a gradual increase 

 in the strength of the solution in which the plant is 

 growing (Experiments). 



The cause of this gradual decrease in absorbing 

 power is obvious on referring back to Experiments 6 

 and 7, where it is seen that, if the solutions in the soil 

 are of a higher density than the cell-sap, then the ^ 

 former cannot be osmotically attracted by the latter;' > 

 that is, root-absorption cannot take place. 



It therefore follows that the plant must take in an 

 excess of water in order to get enough of the mineral 

 foods which it obtains from the sou, as the mineral 

 solutions are necessarily weak. 



Other advantages are also derived by the plant from 

 the transpiration current, as the rapid flow of water 

 through the plant is termed. One important gain is 

 the lowering of the temperature of the leaf during hot 

 weather ; such reduction of temperature being brought 

 about by the evaporation of Avater from its surface. 



Leaf-fall. — The connection between absorption and 

 transpiration at once throws light on the question of 

 the falling of leaves in autumn. The coming of winter 

 means a decrease in the power of absorption by the 

 roots, since it has been proved that absorption decreases 

 as the cold increases. Transpiration decreases also, 

 but, as has been seen, the power of a plant to prevent 

 loss of water is only a limited one. The tree must 

 therefore husband the water that it can obtain until 

 warmer conditions bring back a renewed power of ab- 

 sorption. This it does by dropping its leaves, since it 

 is through the leaves that the water would otherwise 

 be lost. 



Summary. — Something has now been learnt of the 

 food that a plant gets from the soil. 



