THE PLANT. 37 



The roots of plants contain sap, which is separated 

 from the plant-food in the soil, by a thin film of 

 matter, which constitutes its cell-walls. So long as 

 the water of the sap has the capacity to dissolve 

 more mineral matter than it already contains, it will 

 take it through the cell-walls, as the salt is taken 

 through the bladder. If the plant-food outside of 

 the roots is in a moist condition, it will be taken up 

 more rapidly than if too dry. The moisture of the 

 soil itself, containing mineral matter in solution, 

 passes through the cell- walls to supply the place of 

 that which has been evaporated at the leaves, the 

 matters in solution passing through with the water 

 itself. 



In short, there is a constant tendency to supply 

 the deficiency of water in the root, and to keep it 

 constantly charged with as much as it can dissolve 

 of the plant-food, from which it is separated only by 

 its membranous cell-walls. 



Under the influence of daylight, the carbonic acid 

 is decomposed ; its oxygen returned to the atmos- 

 phere, and its carbon retained in the plant. 



The water taken in by the roots circulates through 

 the sap vessels of the plant, and is drawn up towards 

 the leaves, where it is evaporated. This water con- 

 tains the nitrogen and earthy food required by the 

 plant and some carbonic acid, while the water itself 

 consists of hydrogen and oxygen. 



Thus we see that the plant obtains its food in the 

 following manner : 



