30 PLANT STRUCTURE AND GROWTH. [cHAP. 
tain iron, nitrogen, potash, phosphorus, and sul- 
phur; they abound in most soils, but cannot be 
taken up by the plant in a solid form, hence, if a 
plant be kept without water, it cannot obtain any 
mineral salts. The gaseous food is obtained from 
the atmosphere through the stomates of the leaf. It 
consists of carbonic acid gas, which is composed of 
carbon and oxygen. The chlorophyll-bearing cells 
have the power of retaining the carbon and setting 
the oxygen free; but it can only do so when under 
the influence of sunshine. On entering the stomates, 
the gas circulates through the intercellular spaces to 
the chlorophyll cells, and through the spiral vessels 
to the fibro-vascular bundles, 
As the roots absorb the liquid food (sa) from the 
sround, it passes from cell to cell, and through cer- 
tain vessels of the fibro-vascular bundle until it 
reaches the leaves. The warmth of the sun causes 
the water in the leaf-cells to evaporate—that is, to fly 
off through the intercellular spaces and the stomates 
in the form of a light vapour. If a plant exposed to 
the sun be not well supplied with water at the roots, 
it will quickly fade and die, in consequence of this 
great evaporation robbing the cells of their moisture. 
As the water evaporates, it is perfectly pure, all the 
mineral substances it previously contained having 
been left in the cells. The carbon absorbed from 
the air is combined with the cell-sap, and forms a 
substance called starch, Of this starch cellulose for 
the cell-walls is formed, and it is also changed by the 
protoplasm into sugar and fat. By the addition of 
nitrogen and sulphur (taken up in water by the 
