GENERAL CONSIDERATIONS. 9 



de Courval first clearly demonstrated its importance, 

 while his discovery of the value of coal-tar or the 

 refuse from gas-works as a 

 covering for wounds made 

 in pruning renders the ap- 

 plication of his rule in all 

 cases entirely safe. 



Formation of Wood. The 

 reason that a branch should 

 be cut close and even with 

 the trunk is found in one 

 of the simple laws of plant 

 life. It is known that sap 

 has a double movement, 

 that it mounts from the 

 roots to the leaves, and 

 returns again in an elabo- 

 rated condition to the roots. 



RootS take Up Water from ^3.-Portion of the tnink of an 



unpruned Oak ruined by the decay of its 



the soil in which there are lower branches, 

 various salts in solution. This water rises to the 

 leaves ; these absorb from the air and decompose 

 carbonic acid gas, the basis of which is carbon, 

 which combined with water constitutes the elements 

 of wood. The sap thus elaborated by the leaves 

 is carried down again in a liquid state and is 

 deposited, year after year, in the successive concen- 

 tric layers of wood which form the trunks of all 

 trees, with the exception of Palms, Yuccas, &c., 

 which need not now be considered. 



