THE STE.M AXl) THE LEAF -Jl 



arrival of cold weatlier, but it orr-nrs just as certaiiily, and 

 often after a briefer period of growth, in regions where there 

 is no cold winter. 



48. Annual thickening. In stems .such as those of dicotyle- 

 donous trees and the trees of the Pine family and other cone 

 bearers, A\-hich lise for many yeais, the cambium forms a ne^w' 

 layer of bark and of wiKjd e^'ery year.^ These annual layers are 

 usually more noticeable in the wood than in the bark, because 

 the wood cylinders thus fonned remain closely joined together 

 (Fig. 35). The newer lighter-colored portions of the wood are 

 known as mpwood, and the older portions, often darkened by 

 the deposit of coloring matter, are known as ]ij;artic(jijd. Xot 

 infrequently the hea^t^\•ood decays and leaves the tree hollow. 



(1) How old is the stick of ^^■ood shown in Fig. 3-5 ? ('!) 

 Did it grow equally fast during eaeli year of its life ? Dis- 

 cuss this question. (3) Wliy is the name " annual rings " not 

 an accurate one? A\'hat are theyTeally? (4) Is each year's 

 growth uniform all round the stem? (-5) Had this stem any 

 branches in the portion shown by the section ? How could 

 the age of the stem, at the time when a branch began, be 

 known (Fig. 37)? 



The hardwood trees show great differences in the rate at 

 which their trunks increase in thickness. Poplars, basswoods, 

 \\illows, or red oaks, growmg in good soil and unshaded, may 

 for forty or fifty years form annual rings as much as three 

 eightlis of an inch thick. Bttt old Ijeeches and sugar maples 

 in the forest, after they have passed the hundred-year limit, 

 often grow not more than about one sixteenth of an inch per 

 year. AVhen very old, though stdl sound, they may grow only 

 about one twenty-fifth of an inch per year. 



Two of the most important of our coniferous or needle- 

 leaved timber trees are the white pme and the long-leaf 

 pine. A white-pine tree, overtopping most of its fellows in 

 the forest, is, on the average, at ten years 0.9 inch in diameter, 



1 In the tropical regions, where there is no marked change of seasons, the 

 wood often grows rather evenly all the year round. 



