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 fibre-vascular bundles. 



As the roots absorb the liquid food (sap) from the 

 ground, 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 



