THE STEM AND THE LEAF 



67 



The hard-wood trees show great differences in the rate at 

 which their trunks increase in thickness. Poplars, bass woods, 

 willows, and red oaks, growing in good soil and unshaded, may 

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

 eighths of an inch thick, but old beeches 

 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. When very old, 

 though still sound, they may grow only 

 about one twenty-fifth of an inch per year. 



Would it be good policy to let beeches 

 and maples remain long in the forest 

 after they are one hundred years old 

 before cutting for timber ? Why ? 



65. Growing points. The extreme tip 

 of the live stem or root of a dicotyle- 

 don consists of a more or less conical or 

 cushion-shaped mass of tissue composed 

 of thin-walled cells like those of the cam- 

 bium layer. This portion of the stem or 

 root is called the growing point. Every 

 live twig and rootlet is tipped with a 

 growing point, and it is by the rapid 

 sub-division and consequent multiplica- 

 tion of these cells that the lengthening 

 of the main stem and its divisions, and 

 of every root, takes place. 



All branches originate from growing 

 points, which are usually developed along 

 leaf -bearing portions of the stem, each one just above the point 

 where a leaf is attached. In their earliest beginnings both 

 leaves and rudimentary branches consist wholly of thin-walled 

 cellular tissue. Fibrovascular bundles, connected with those 

 of the underlying stem, soon appear in the branches and leaves 

 as their development goes on. 



FIG. 46. Formation of 

 knots due to branches. 

 The figure gives part of 

 a lengthwise section of 

 a stick of birch wood 



a, section of the base of 

 a branch which persisted 

 until the tree was felled ; 



b, section of the base of a 

 branch which died some 

 years earlier and is now 

 covered by several layers 



of younger wood 



