82 THE MECHANISM OP LIFE 



this consists of about 79 per cent, of nitrogen, 21 per cent, of 

 oxygen, and a trace of carbonic acid. If we next examine the 

 air of an isolated space which has been used by plants, we shall 

 find that its percentage of carbonic acid has been greatly dimin- 

 ished, while that of oxygen has increased. Evidently the 

 effect of plant life is to rob the atmosphere of its carbonic acid 

 and to enrich it with oxygen — just the opposite to that of animal 

 life. If, further, we proceed to examine the water which is taken 

 up by the roots, we shall find that this is not pure, but always con- 

 tains mineral substances like nitrates, chlorides of potash and 

 magnesium, salts of iron and lime, phosphates, silica, sulphates, 

 etc., and it can be proved that the effect of plant metaboHsm is 

 to take such substances from the soil water. 



The materials from which the green plant builds up its tissues 

 are therefore water, carbonic acid, nitrates, and other simple 

 mineral substances. These materials it converts into proteids, 

 oils, and waxes, and carbohydrates such as starch, sugar, and 

 cellulose. Now all of the latter substances contain much poten- 

 tial energy, for, if we dry them, we can burn them and so obtain 

 heat. But the water, carbonic acid, and mineral salts cannot be 

 oxidised further, and they contain no available energy. In 

 building up its tissues the green plant must therefore obtain 

 energy from somewhere, and it can be proved that it is obtained 

 from light ; if the plant is kept in the dark, there can be no growth.* 

 But it can grow in the light radiated from an electric arc. It is, 

 in fact, fairly easy to show that a green leaf exposed to sunlight 

 is continually forming starch, while, if it is kept in the dark, no 

 such thing happens. 



Here, then, we have the source of energy of the green leaf. 

 Outside the latter is water (OHg), carbonic acid (COg), and solar 

 radiation; and inside it is the mixture of pigments called 

 chlorophyll. The solar radiation is not heat, although it is 

 transformed into heat when it impinges upon most material 

 objects. When it falls upon certain substances, such as luminous 

 paints, it is transformed into light, and in certain conditions it 

 can be transformed directly into mechanical work. When it 

 impinges on the green leaf and is absorbed by the chlorophyll 

 pigments, it is immediately transformed into chemical energy; 

 for it can be proved that there is no starch in a leaf which is kept 



* No increase in mass, although a seed — a potato, for instance — may 

 germinate in the dark. 



