STEM AND BRANCHES. 



15 



in cross-section a central colored portion, termed heart-wood or duramen, 

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

 riving 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 the foregoing it becomes evident that the proliferation of wood- 

 cells is only to be found in progress upon the outside of the wood already 

 formed, just underneath the bark. As aggregated here, these young grow- 

 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. 



FIG. 24. Vertical section of the stem of a maple, showing the medullary rays. Magnified. 



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 upon its 

 outer surface, the bark thickens by deposition of new material upon its 

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

 ward, the bark is as constantly dying and exfoliating from without in- 

 ward. Moreover, as the stem increases in size the outer bark, 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 rapidly 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. 



