Correspondence Courses in Agriculture. 5 



The shape, size and texture of leaves vary greatly in different 

 species, often in the same tree, as a result of the struggle for ex- 

 istence for light and air. All gradations of size and shape occur, 

 from the broad, soft leaves of cottonwood (Paulonia) to the needle- 

 like leaves of pine and the sharp, scale-like leaves of red cedar. 



The form of crown, as well as of leaf characterizes the different 

 species, such as the inverted- vase form of elm, the conical form of 

 hemlock, the rough, irregular form of oak. 



An intimate relation exists between the size of the crown and the 

 extent of the root-system. If for any reason the one is not allowed 

 its full development, the other will be correspondingly affected. 

 Since the amount of food manufactured, and consequently the 

 amount of growth, depend upon the extent of leaf surface in the 

 crown, it follows that any operation which will result in more light 

 and a greater growing space for the crown will influence the rate of 

 growth favorably. This fact is of great importance in the practice 

 of forestry and for this reason, the crown becomes of special inter- 

 est in the management of the woodlot. 



Structure of Wood. The stem, branches and roots of trees are 

 composed of woody tissue. This is not a simple substance like iron 

 or gold, but as is true of all organic material, is composed of minute 

 cells. These are somewhat similar in appearance to the cells of 

 honeycomb, though many times smaller. In a cross-section of oak 

 the centers of these cells may be seen as tiny holes. The walls of 

 the cells are built up of cellulose and lignin, the main constituent 

 of which is carbon. In the living state these cells are more or less 

 filled with a living substance (protoplasm), which is common to 

 all life. The cells are not all alike but differ in form, size and 

 thickness of wall, and in their use. Some conduct food material 

 to and from the crown, some store reserve food material until it 

 is needed, others serve simply to strengthen the woody tissue. 



Parts of the Stem of the Tree. If the top of a stump of a recently 

 cut oak is examined, it will be found to be made up of several parts. 

 Around the outside is observed the bark, which is composed of two 

 parts, the outer, protective, corky layer and the inner, light-colored, 

 softer layer. Inside the bark is found the wood proper, which is 

 likewise differentiated into two parts, the outer, lighter-colored, 

 moist "sap wood" and the inner, dark-colored, firmer "heart-wood." 

 At the center is found the pith, very inconspicuous in most trees. 

 From the bark to the pith are found broken, wide or narrow lines 

 of woody tissue, known as medullary rays. To the medullary rays 

 is given the credit for the beautiful silver-grain of quarter-sawn oak. 

 They carry food from the bark to the inner portions of the tree, 

 serve as a storehouse fop surplus food material, and form a means 

 of communication with the air. Occurring in all species of wood, 

 of*;en so small as to be invisible to the naked eye, thousands may be 

 counted to the square inch. In a few species, as birch and cherry, 

 their termination in the bark is marked by conspicuously rough- 

 ened spots known as "lenticles." 



Between the bark and 'the "sap-wood" is found a layer of wet, 



