124 THE VEGETABLE KINGDOM. 



amount of carbonic acid and ammonia, which the air always 

 contains, and this is the whole source of the nitrogen 

 which forms a very important part of the bodies of plants 

 and animals. When the rain arrives at the surface of the 

 earth, it sinks down into it and carries with it all soluble 

 vegetable or animal matter which it meets with, together 

 with any soluble earthy matter which may exist in the soil ; 

 this forms the sap of the tree. When it arrives at the 

 surface of the leaf, the watery part of it combines with the 

 carbonic acid of the air (through the influence of light) and 

 appropriating its carbon, gives out the oxygen ; this is the 

 true respiration of plants, and is exactly the reverse of what 

 takes place during the respiration of animals, in which case 

 oxygen is absorbed and carbonic acid given off. The carbon 

 thus retained by the plant combines with the elements of the 

 water to form the solid green substance called chlorophyll, 

 which is the basis of all the tissues of the plant, the ammonia 

 is also decomposed, and its nitrogen combining with the 

 oxygen and hydrogen of the water, and the carbon of the 

 carbonic acid forms those compounds which constitute the 

 most nourishing parts of vegetables, such as albumen, gluten, 

 &c., and of which all the animal tissues are built up, for the 

 production of these organic substances takes place in the 

 vegetable only, animals simply appropriating them from 

 their food. The sap which reaches the leaf is not all con- 

 verted into chlorophyll, but also into those peculiar juices 

 which are found in plants, some of which contain sugar, 

 some gum, others (as the pine tribe) turpentine, and in the 

 laurel tribe camphor, all of which are substances containing 

 much carbon ; moreover the solid wood and bark are deposited 

 from these juices as they descend from the leaf after having 

 been acted on by light (or the actinic power associated 

 with it). Now, as the water, ammonia, and carbonic acid 

 which descend with the rain are from the air, and as the 

 vegetable is formed wholly by their absorption, it may be 

 fairly said that the vegetable kingdom (and therefore the 

 animal) feeds upon the air, and that the trees do not grow 

 out of the earth but into it. 



With respect to the position which the vegetable kingdom 



