36 THE STOHY OF THE 



Now, what is a leaf ? It is usually a rather 

 thin, flat body, often with two parts, a stalk and 

 a blade, as in the oak or the beech ; though 

 sometimes the stalk is suppressed, as in grass 

 and the teasel. Almost always, however, the 

 leaf is green : it is broad and flat, with a large 

 expanded surface, and this surface is spread out 

 horizontally, so as to catch as much as possible 

 of the sunlight that falls upon it. Its business is 

 to swallow carbonic acid from the air, and digest 

 and assimilate it under the influence of sunlight. 

 And as different situations require different 

 treatment, various plants have leaves of very 

 different shapes, each adapted to the habits and 

 manners of the particular kind that produces 

 them. The difference has been brought about 

 by Natural Selection. 



What does the leaf eat ? Carbonic acid. 

 There is a small quantity of this gas always 

 floating about dispersed in the air, and plants 

 fight with one another to get as much as possible 

 of it. Most people imagine plants grow out of 

 the soil. This is quite a mistake. The portion 

 of its solid material which a plant gets out of 

 the soil (though absolutely necessary to it) is 

 hardly worth taking into consideration, nume- 

 rically speaking ; by far the larger part of its 

 substance comes directly out of the air as 

 carbon, or out of the water as hydrogen and 

 oxygen. You can easily see that this is so if 

 you dry a small bush thoroughly, leaves and 

 all, and then burn it. What becomes of it 

 in such circumstances ? You will find that 

 the greater part of it disappears, or goes off 



