242 A YEAR IN AGRICULTURE 



of the tree, as well as of all plants, are to get nourishment 

 and to reproduce its kind. The tree gets its food from the 

 soil, and the air through its roots and leaves. The solid food 

 from the soil must go into solution and be carried upwards 

 from the roots through the sap-wood to the leaves. The gas 

 food must be taken in through the leaves. All this food, the 

 minerals from the soil and the carbon from the air, is pre- 

 pared for the different parts of the tree in the leaves by the 

 aid of the sunlight. The prepared food is then carried down- 

 ward through the inner soft bark to where it is needed to 

 make root, trunk, branch, leaf, flower, and fruit. Girdling 

 a tree, therefore, checks this downward flow of food and not 

 the upward flow of crude sap. 



The trees, except those of the palm tribe, grow in girth 

 by adding ring upon ring of wood cells to their trunks and 

 branches; in height, not by lifting the whole trunk and 

 crown, but by adding to the tips of the twigs. Trees repro- 

 duce by seeds, sprouts, and sometimes by cuttings. 



Structure. A tree, like every other living thing, is com- 

 posed of tissues made up of minute cells varying in shape, 

 size, and thickness of cell wall. The bulk of the bole of the 

 tree is not living but dead tissue, composed of empty cells. 

 For this reason the heart of a tree may be dead and the tree 

 continue to live and grow. The living part of the tree trunk 

 is on the outside of the wood, between bark and wood. The 

 growing tissue of this live part is called the cambium. Grow- 

 ing cells are also grouped at the tips of roots and at the tips 

 of the shoots. The thick outer bark of the tree is dead tissue 

 which sooner or later loosens and sloughs off. In the center 



