86 



GENERAL BOTANY 



In sections cut across more mature stems of trees like the oak 

 and the alder the above layers will be found to be considerably 

 changed in their relative width and structure (Fig. 47). In such 

 sections the outer, brown bark is much thicker and is often cracked 

 or seamed by the great increase in the diameter of the central 

 wood cylinder. The green bark has also usually disappeared 

 in mature stems and forms a thin layer of crushed cells be- 

 tween the outer corky bark and the inner light bark, or phloem. 



Cork bark 



Skeletal tissue (fibers 

 and sderenchyma) 



Phloem 

 Cambium 

 Summer wood 

 Spring wood 



Wood ray 



FIG. 47. Transverse section of an oak stem eight years old. Diagrammatic 



The central darker dotted portion is heartwood ; the outer wood layers, with alter- 

 nating light spring wood and dark summer wood, belong to the sap wood 



The wood cylinder is also differentiated into an outer light area 

 of sapwood composed of several of the latest-formed annual rings 

 of growth, and of a darker central portion, the heartwood, made 

 up of the first-formed wood in the center of the tree. The sap- 

 wood contains living cells in the form of wood rays and wood 

 parenchyma and is active in the conduction of water and food 

 and in the storage of food reserves. The heartwood is dead and 

 serves a purely mechanical function in supporting the tree. Its 

 cell walls are impregnated with various substances which change 

 their color and increase their strength. 



