16 Leading Principles of tlie Growth of Trees. 



pear have about 25,000 or 30,000, and the white lily about 60,000 

 to the square inch. They are mostly on the lower side of the leaf. 

 Fig. 5 represents the pores on an apple-leaf. Leaves are a contri- 

 vance for increasing the surface exposed to the air and sun 

 Prof. Gray says the Washington elm at Cambridge was es- 

 timated to bear " seven million leaves, exposing a surface 

 of 200,000 square feet, or about five acres of foliage." A 

 common fully grown apple-tree has from three to five hun- 

 dred thousand leaves, and the breathing pores they all con- 

 Fig. s> tain must be more than a thousand million. 



THE PROCESS OF GROWING. 



Water is absorbed by the roots, and undergoes a very slight 

 change ; matter from the cells of the root is added (as sugar, in the 

 maple), and it is then denominated sap. It passes from cell to cell 

 upwards, 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 bass- 

 wood, for example, which has very long cells, it passes about 

 2,000 partitions 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 becomes thickened. The carbonic acid of the air, and 

 the small portion of the same acid which the sap contained before it 

 entered the roots, now forms a combination with the oxygen and 

 hydrogen of the sap, and produces the triple compound 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 appearance. If continued, air-bubbles rise in the water, 

 and if a glass tumbler full of water is inverted over them, pure oxy- 

 gen 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 atmosphere. Fires in burning, 

 and animals in breathing, consume carbon, combine it with oxygen, 

 and then throw off the carbonic acid thus formed. This acid, being 

 poisonous, would after a while become so abundant as to prove 

 injurious to animal life, were it not for the wise provision by which 



