160 THE PLANT IK ACTION". [LESSOR 26. 



sue, in the tender pot-herb and in the oldest tree. It is composed 

 of carbon, hydrogen, and oxygen, 6 parts of the first to 10 of the 

 second and 5 of the third. These, accordingly, are necessary mate- 

 rials of vegetable growth, and must be received by the growing plant 



455. The Plant's Food must contain these three elements in some 

 shape or other. Let us look for them in the materials which the 

 plant is constantly taking from the soil and the air. 



456. Water is the substance of which it takes in vastly more than 

 of anything else : we well know how necessary it is to vegetable life. 

 The plant imbibes water by the roots, which are specially construct- 

 ed for taking it in, as a liquid when the soil is wet, and probably 

 also in the form of vapor when the soil is only damp. That water 

 in the form of vapor is absorbed by the leaves likewise, when the 

 plant needs it, is evident from the way partly wilted leaves revive 

 and freshen when sprinkled or placed in a moist atmosphere. Now 

 water is composed of hydrogen and oxygen, two of the three elements 

 of cellulose or plant-fabric. Moreover, the hydrogen and the oxygen 

 exist in water in exactly the same proportions that they do in cellu- 

 lose : so it is clear that water furnishes these two elements. 



457. We inquire, therefore, after the third element, carbon. This 

 is the same as pure charcoal. Charcoal is the carbon of a vegetable 

 left behind after charring, that is, heating it out of contact of the air 

 until the hydrogen and oxygen are driven off. The charcoal of wood 

 is so abundant in bulk as to preserve perfectly the shape of the cells 

 after charring, and in weight it amounts to about half that of the 

 original material. Carbon itself is a solid, and not at all dissolved 

 by water : as such, therefore, it cannot be absorbed into the plant, 

 however minute the particles ; only liquid and air can pass through 

 the walls of the cells (402, 410). It must therefore come to the 

 plant in some combination, and in a fluid form. The only substance 

 within the plant's reach containing carbon in the proper state is 



458. Carbonic Acid. This is a gas, and one of the components 

 of the atmosphere, everywhere making about ^^ part of its bulk, 

 enough for the food of plants, but not enough to be injurious to 

 nnimals. For when mixed in any considerable proportion with the 

 air we breathe, carbonic acid is very poisonous. The air produced 

 by burning charcoal is carbonic acid, and we know how soon burning 

 charcoal in a close room will destroy life. 



459. The air around us consists, besides this minute proportion 

 of carbonic acid, of two other gases, mixed together, viz. oxygen 



