vm.j OF THE STEM. 103 



answering to one of the annual zones, or may more 

 gradually and irregularly graduate into it. The inner 

 more deeply coloured wood is termed the heart-wood or 

 duramen, the outer younger and paler the sap-wood or 

 alburnum. It is probable that the distinction between 

 heart-wood and sap-wood is primarily "due to a fermenta- 

 tive process which probably affects the molecular com- 

 position of the cell-walls of the former. Many of our 

 cabinet-woods derive their ornamental value from this 

 change in their heart-wood. 



With a little care you may notice that there are, as it 

 were, narrow rays proceeding from the pith to the bark. 

 These are actual plates of unmodified primitive cellular 

 tissue left between the fibro-vascular bundles, which look 

 like narrow rays when the stem is cut through trans- 

 versely. They are called the medullary rays. They are 

 usually very narrow, much narrower in most trees than in 

 the Oak, the wood of which, when 'cut lengthwise in the 

 plane of these rays, is marked by silvery patches of the 

 cells of the rays, forming what is called the silver-grain, 

 which house-painters imitate in painting wainscot. 



It will be difficult to find the cambium-ring without 

 using a lens. It is a narrow belt of thin-walled cells, 

 easily ruptured in spring, immediately within the bark, 

 which it connects with, while at the same time it separates 

 it from, the wood. 



The outer layers of bark are usually composed of 

 short cells of corky texture, partly the remains of the 

 primitive cellular tissue of the axis before the formation 

 of vascular bundles within it. These corky cells serve 

 to prevent the cambium-layer from drying up, by checking 

 evaporation from the surface. 



From the mode of increase in diameter of the woody- 

 axis of Dicotyledons indicated above (p. 101), it must 

 follow that unless the outer bark retains sufficient 

 vitality to permit the formation of new cells, so as to 

 accommodate itself to the increasing diameter of the 

 wood which it incloses, it must necessarily become 

 ruptured. And so we find in the majority of our trees 

 that the bark becomes longitudinally fissured, as in Oak, 

 Elm, and Willow. In healthy Beech, however, the bark- 

 retains sufficient vitality to permit dilatation part passu 



