PRINCIPLES OF THE GROWTH OF TREES. 9 



sap. It passes from cell to cell upward, through the sap- 

 wood, until it reaches the leaves. The cells being separate, 

 and not continuous tubes, it is conveyed from one to another 

 through a great number of partitions ; in the basswood, for ex- 

 ample, which has very long cells, it passes about 2,000 parti- 

 tions in rising a foot. 



When the sap enters the leaf, it emerges from the dark cells 

 through which it has been passing, and is spread out to the 

 light of the sun. A large portion is evaporated through the 

 breathing pores, and it bec6mes thickened. The carbonic 

 acid of the air, under the influence of sunlight and in the pres- 

 ence of the leaf-green, now forms a combination with the 

 oxygen and hydrogen of the sap, and produces the triple com- 

 pound of oxygen, hydrogen, and carbon, which constitutes 

 woody fibre the oxygen of the carbonic acid escaping. This 

 escape of oxygen may be seen by placing leaves under water 

 in the sunshine. Innumerable little bubbles of oxygen form 

 on the surface of the leaves, and give them a silvery appear- 

 ance. If continued, air-bubbles rise in the water, and if a 

 glass tumbler full of water is inverted over them, pure oxygen 

 in small quantities may be procured. A plant growing in car- 

 bonic acid gas takes the carbon, and leaves the oxygen ; in 

 this way changing the acid to oxygen. Growing plants thus 

 perform a most important office by purifying the atmopshere. 

 Fires in burning, and animals in breathing, consume carbon, 

 combine it with oxygen, and then throw off the carbonic acid 

 thus formed. This acid, by excluding oxygen, might after a 

 while become so abundant as to prove injurious to animal 

 life, were it not for the wise provision by which plants con- 

 sume it and restore the oxygen. Connected with this, there 

 is another interesting proof of creative design. If there were 

 no carbonic acid in the air, plants could not grow ; but one- 

 twenty-five-hundredth part, as now exists, supplies food for 

 vegetation, and does not affect the health of animals and man. 



Leaves require sunlight to enable them thus to decompose 

 carbonic acid. It does not go on in a dark room, or in the 

 night. Anything which excludes light and air tends to make 

 the product of the plant inferior or bitter. The fruits cannot 

 ripen fully. Their juices do not reach their full chemical ma- 

 turity. Thinning and pruning are therefore important. 



