PRINCIPLES OF THE GROWTH OF TREES. $ 



THE STEM AND BRANCHES. 



As roots are annual, biennial, or perennial, as they continue 

 living one, two, or more seasons ; so the stem is herbaceous or 

 woody, as it grows only one year or more in the latter in- 

 stance hardening into wood. Woody plants, when small, are 

 called shrubs, as the rose, gooseberry, and currant. When 

 large, they are trees, as the apple, pine, and oak. A dwarf 

 apple, made small by budding any common variety on the 

 small Paradise stock, becomes a shrub. Suckers are branches 

 springing up from underground stems; some times they come 

 from mutilated roots. Runners are creeping stems, which 

 strike roots at the tips and form leaves there, as in the straw- 

 berry. A single strawberry plant will in this way produce a 

 hundred new ones or more in a summer; and by care ten 

 thousand by the end of the second year, a million the third, 

 and so on. 



Stated in general terms the stems or trunks of hard wood 

 trees (dicotyledons) are formnd of bark, cambium layer, wood, 

 and pith. The outer bark on some trees gradually forms into 

 a thick, hard, corky substance, termed cortical layers, but while 

 young it is the green bark of growing shoots. The inner layer 

 of bark, next the cambium, is called the bast layer or liber, 

 from the resemblance of the concentric plates of which it is 

 formed to the leaves of a book. 



The cambium layer is the active, cellular agent in the 

 growth of the tree. It lies between the bark and wood. 

 From its inner surface is produced the growth of wood, and 

 from its outer the bark is formed. Thus the newest bark is 

 inside, and the newest wood outside. 



Wood. The outer wood, which is the youngest and freshest, 

 is called the alburnum or sap-wood. The heart-wood is the 

 older, harder, and usually more dried portion ; and it bears 

 the same relation to the sap-wood as the cortical layers do 

 to the liber. 



The pith, in young plants, performs a useful office by re- 

 taining moisture ; but in old trees it becomes dry, shrivelled, 

 and useless, and trees grow as well where it has been cut out. 



Branches. These consist of main branches, or limbs; second- 

 ary or smaller branches; and s/wots, or the extremities, being 

 one year's growth. Thrns are usually a modification of 



