THE PIXE 119 



cells and bark are formed, and the older layer of bark peels 

 off. Then a third layer of cork cambium appears within the 

 second, then a fourth, and so on, and in time the whole cortex 

 has been changed to bark. Later laj^ers of cork cambium 

 therefore must be formed within the bast. So the outer part 

 of the bast is also gradually changed to bark and scaled off, 

 at the same time that new bast is being formed from within 

 by the division of the cambium cells. This change of the 

 outer layers of the bast into bark explains why the bast never 

 becomes thick, although new bast is formed each year ; and 

 why most of the thickness of an older tree is made up of rings 

 of wood. 



144. Branches. — These, in the pine, are of two kinds : 

 long branches and spiir branches. The long branches develop 

 chiefly from the buds borne in the axils of winter bud scales ; 

 but sometimes adventvce hvids, which appear in various places 

 — • for example, upon wounded parts of a trunk or branch — 

 grow into long branches. A long branch has the same struc- 

 ture as the trunk ; it grows in length and in thickness ; it 

 forms bark ; and from the axils of its ^^-inter bud scales new 

 long branches may grow. The spur branches are ver}- short, 

 never more than a fraction of an inch in length. They grow 

 from the buds in the axils of the numerous scale leaves that 

 are formed during the growing season upon the trunk and the 

 long branches. A spur branch hves and grows xerx slowly 

 for several years, never itself branching. At its end is a 

 cluster of green foHage leaves. 



145. Leaves. — We have seen that two kinds of leaves are 

 borne by the pine : scale leaves ,( which include the winter 

 bud scales) and foliage leaves. Scale leaves are protective 

 organs and are usually short-lived. Much more conspicuous 

 are the long, green foliage leaves — the " pine needles " — 

 which grow in clusters at the ends of the spur branches. 

 Each cluster is surrotmded in the bud by a thin membrane, 

 which later is torn and partly or entirely disappears. The 



