THE YOUNG NATUEmST. 



S69 



Thus they iuhale the atmospheric air, 

 retaining a certain amount of the 

 oxygen which is essential as the hfe 

 sustaining principle, alike in plants 

 and animals, and they exhale the 

 residue laden with a certain degree 

 of impurities. This process is always 

 most apparent at night, or during 

 darkness, hence the old prejudice 

 against having flowers in bedrooms, 

 but it has been shown that under 

 all circumstances the air in the neigh- 

 bourhood of growing plants is always 

 the freest from substances inimical 

 to animal existence. It is now well 

 known that the breathing of animals 

 vitiates the air by throwing into 

 it a quantity of carbon dioxide, which 

 augmented by the combustion of 

 fires contaminates the air to the 

 extent of four in every ten thousand 

 parts, and it is from this ap- 

 parently small quantity, that the 

 great bulk of all plants is derived. 

 This process known as assimilation 

 is entirely performed by the leaves. 

 The ordinary air is inhaled by the 

 leaves, and the quantity of carbon is 

 greedily absorbed by the chlorophyll, 

 which is the green colouring matter of 

 leaves, and the life sustaining oxygen 

 is thus purified and liberated to take 

 part again in the vital economy of 

 nature. The activity of this process 

 is dependent upon light, and is most 

 vigorous in direct sunshine. A 

 beautiful and simple experiment will 



illustrate this evolution of oxygen, if 

 a small piece of a healthy growing 

 water plant, such as the water thyme 

 [Elodea), or a branch of fresh water 

 cress will do, be immersed in a glass 

 vessel filled with spring water, which 

 always contains carbon dioxide, and if 

 it be placed in the full rays of the sun, 

 a continuous stream of small bubbles 

 will be observed escaping from the cut 

 portion of the plant, these air bells are 

 pure oxygen set free by the vital action 

 of the plant. The indispensable effect 

 of light is very evident in the pale and 

 colourless appearance of plants grown 

 in comparative darkness, as seen in the 

 blanched leaf stalks of earthed up 

 celery. The leaves are therefore the 

 chemical laboratory in which is first 

 elaborated the plastic material out of 

 which all the tissues of the plant are 

 formed, thus the crude sap which 

 is absorbed by the roots is carried up 

 through the stem, and spread out in 

 the leaves to the influence of the air 

 and light, and it there becomes changed 

 into the peculiar juices characteristic 

 of the plant. These formative materials, 

 however, are never allowed to remain 

 in the leaves or workshops, but they are 

 constantly being removed to the place 

 where growth is going on ; or if not 

 required for immediate use, they may 

 be stowed away in some store-house 

 as a reserve supply for the future need 

 of the plant. Thus the familiar tuber 

 of the potato, or the enlarged and 



