52 



PRACTICAL BOTANY 



at one hundred years 17.2 inches, and at two hundred years 31 

 inches. The average thickness of the " annual rings " during 



the life of the tree throughout 

 its second century is therefore 

 about one fourteenth of an 

 inch. In the Southern long- 

 leaf pine, growth is slower. 

 The increase in thickness of a 

 tree two hundred and twenty 

 years old and 17| inches in 

 diameter was only one inch 

 during the last forty years, 

 or one fortieth of an inch 

 per year. 



In successful white-pine 

 trees (that is, the taller and 

 only slightly shaded ones of 

 a forest) the total amount of 

 wood formed in the trunk 

 per year is, at fifty years, 

 about one fourth of a cubic 

 foot, at sixty to seventy years 

 one cubic foot, at one hun- 

 dred years one and one half 

 cubic feet. 1 



49 o Origin of branches. The 

 branches of dicotyledons be- 

 gin as little elevations or 



FIG. 36. Diagrammatic section through 



the growing tip of a dicotyledonous 



shoot, showing origin of branches 



g, the growing tip of the shoot ; I, leaves, 

 those at the upper part of the shoot the 

 youngest; 6j, 6 2 , 63, 64, branches of 

 various ages, arising in the axils of the 

 leaves. Note that only the older leaves 

 and branches have fibrovascular bundles, 

 connecting with those of the main portion 

 of the shoot (all deeply shaded in the 

 diagram) . After Luerssen 



rounded outgrowths from the 

 axis of the leaf bud, which often terminates the stem. 2 The 

 extreme tip of the stem is the growing point (</, Fig. 36). This 

 and much of the neighboring region is made up of cells which 



1 See "The White Pine," Bulletin^ Division of Forestry, U. S. Dept. Agr. 



2 Buds are treated more in detail in Chapter VI, but it seems best to say 

 a few words in this place about the relation of the beginning branch to the 

 axis from which it grows. 



