20 SCIENCE PRIMERS. [v. 



V. THE FOOD OF PLANTS. 

 ABSORPTION, TRANSPIRATION, ASSIMILATION. 



24. The food of plants is partly gaseous and partly 

 liquid; and is derived from the earth or water in 

 which they grow, and from the air. The liquid food 

 is taken into the plants chiefly by their roots, the 

 gaseous chiefly by their leaves. 



25. The gaseous food of plants consists of car- 

 bonic acid gas, supplied chiefly by the atmosphere. 

 The liquid food is water, in which various saline 

 substances are dissolved, the principal components of 

 these being nitrogen, phosphorus, sulphur, potash, 

 and iron. The above-named matters are found in 

 most soils in which plants grow, but cannot be taken 

 up by the roots except they be dissolved in water. 



26. Absorption. The taking in of liquid food 

 by the roots is called absorption, and the liquid 

 absorbed becomes part of the sap in the plant. The 

 sap ascends through the stem and branches, and so 

 reaches the cells of the leaves, or the cells near the 

 surface of plants that have no leaves. In mounting 

 up it passes both from cell to cell through their walls 

 and along some of the tubes of the vascular tissue. 



The taking in of carbonic acid gas from the air is 

 another process of absorption. It is performed by 

 the leaves, in the cells of which a chemical process 

 goes on under sunlight, by which the carbon is broken 

 up. the carbon being retained by the plant, and the 

 oxygen given back to the air. The carbon being at 

 the same time combined with the oxygen and hydrogen 



