2IO THE CULTIVATION OF THE 



in the sap. unites with the oxygen and hydrogen of the 

 sap, and produces wood fibre, which is a triple compound 

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

 carbonic acid escaping. Here the tree in breathing then 

 gives out oxygen, while an animal in breathing, or any 

 fire burning, gives out carbonic acid, and thus has God 

 established the equilibrium in nature — and thus it is 

 that what is poison to man is food to plants. 



Now leaves require sunlight to decompose carbonic 

 acid, and it does not go on in the dark, and, hence, 

 flowers in bed-rooms, at night, are not feeding on the 

 carbon exhaled by sleepers — and doing the good many 

 suppose them to do — but in day time they are in the sun- 

 light actively decomposing carbonic acid gas and giving 

 out oxygen, and, hence, may be tolerated as healthful, 

 so far as this process is concerned, but I discourage, 

 altogether, the keeping of growing plants in any part of 

 dwelling houses, as the earth about them and the decay- 

 ing parts of the plant being necessarily attended by 

 fermentation, disease-germs may be developed with 

 serious consequences. The sap having now been thick- 

 ened by evaporation in the leaf, comes down through the 

 inner bark and forms a layer of semi-liquid matter 

 between the bark and the wood ; this is called cambium 

 and is now divided up, the great part goes to make a 

 new layer of wood, and a small part goes to make a new 

 layer of bark. In budding and grafting this cambium is 



