106 ELEMENTS OF STRUCTURAL BOTANY. 



is no cambium-layer , and consequently no arrangement 

 for the indefinite continuance of the growth of the 

 bundles. Once formed, therefore, they remain un- 

 changed, and the growth of the stem consists in the 

 production of new ones. These (which originate at the 

 bases of new leaves) being introduced amongst the 

 older ones, act as wedges, and swell the stem as a 

 whole. 



THE FOOD OF PLANTS. 



172. A word or two is necessary on this subject in 

 addition to what has already been said. The nature of 

 a plant's food may be determined by making a chemical 

 analysis of the plant's substance. As already stated, 

 the chemical elements found in plants are chiefly four, 

 carbon, oxygen, hydrogen. anCi nitrogen, the latter ele- 

 ment occurring in the protoplasm of active cells. What, 

 then, are the sources from which the plant obtains these 

 materials of its growth ? In the atmosphere there is 

 always present a gas known as carbon dioxide, or car- 

 bonic acid. This gas, which is a compound of carbon 

 and oxygen, \z produced largely in the lunge of animab, 

 ancT by them exhaled. 1^ is readily soluble in water. r:o 

 tha; raiu-dropc in their passage through the air dissolve 

 iu and carry it with them into the soil. Again, wherever 

 aui'jaai or vegetable matter is decaying thece is pro- 

 duced a gas called ammonia, a compound of nitrogen 

 and hydrogen, and, like carbonic acid, readily soluble, 

 HO that thic also is present in rain-water. And when it 

 is considered that :« very large proportion of the air con- 

 sists o^'freo nitrogen, soluble to some extent in water, and 

 that- the elements of water itself arc oxygen and hydro- 

 gen it will be evident that the moisture in the 

 earth contains a 'supply of ever- one of the elemeuta 



