STEM AND BRANCHES. 



15 



in cross-section a central colored portion, termed hcai't-wood or duramen, 

 surrounded by a lighter zone of sap-wood or alburnum, the latter de- 

 rivin"- its name from the fact that the sap still circulates through it, while 

 the former no longer possesses vital activity, and like the pith may, and 

 often does, decay without impairing the vigor of the plant. 



From tlie foregoing it becomes evident tliat the proliferation of wood- 

 ceJls is only to be found in 2)rogres8 ui)on the outside of the wod already 

 formed, just underneath the bark. As aggregated here, these young gi'ow- 

 ing cells form what is termed the cambium layer. This layer is com- 

 monly of a mucilaginous character, and during the period of its greatest 

 activity permits the bark to be readily separated from the stem. 



Fio. 2-1. — Vortionl section of the stem of a niiipk', sliowiiiK' the iiKdullaiy my.). Ma;-rnifiO(l. 



I 



The bark is the protective covering of the stem. At first composed, 

 like the pith, wholly of cellular tissue, it later develops wood-cells, par- 

 ticularly on its inner surface, and is divisible into an inner and an outer 

 layer. The inner bark is commonly composed of long wood-cells, termed 

 bast'Cells, united into fibres often of great strength and toughness, as 

 in flax and hemp. The outer bark is composed largely of cellular tissue, 

 and presents two layers, the inner green, the outer corky ; the latter 

 being covered when young with a delicate epidermis. Like the wood which 

 it covers, the bark increases in thickness each year, but in a reverse 

 manner. That is to say, while the wood increases by growth iipf)n its 

 outer surface, the bark thickens by deposition of new material ui)on its 

 inner side ; and while the wood is continually dying from witliin out- 

 ward, the b.'irk is as constantly dying and exfoliating from without in- 

 ward. INIoreover, as the stem increases in size the outer bai'k, being only 

 moderately elastic, is split and broken, and commonly assumes a rugose 

 appearance. In young and very vigorous trees not unfrequently the wood 

 grows so rajiidly that the bark cannot keep pace with it, and is conse- 

 quently split down to the fibrous layer, or even in some instances to the 

 wood itself. 



