THE GROWTH OP ROOTS AND STEMS. 179 



from the base of the stem to the apex. The rays can often 

 be seen very plainly on a square Pine match ; two opposite 

 faces show the rays as broad, shiny, horizontal streaks (radial 

 faces), while on the other two (tangential) faces they appear 

 as narrow vertical streaks. 



210. Heart- Wood and Sap- Wood. In old trees show- 

 ing many annual rings, the central region of secondary wood 

 is often distinctly marked off from the outer region. This is 

 largely due to the wood-cells losing their contents and to 

 changes in colour and in chemical composition of the walls of 

 the wood- vessels. The central region (heart- wood) is harder 

 and darker than the peripheral region (sap-wood). 



It is almost entirely by means of the young sap-wood that 

 water is conveyed upwards from the roots to the leaves. As 

 the wood grows older it becomes unable to act as a channel 

 for the passage of water. Hence the importance of the 

 addition of new layers of wood is two-fold : first, it provides 

 a connected series of new channels for the ascent of water as 

 the old ones become functionless ; and secondly, it increases 

 the strength of the stem as the weight of the upper portion 

 of the tree increases. 



211. Knots. The knots which occur in wood are branches 

 which have become surrounded by the new layers of wood 

 produced as the stem increased in thickness. Examine logs 

 of various kinds of wood, and find out all you can about 

 the history and mode of formation of the knots. 



Do the knots run right through the tree-trunk from outer 

 surface to centre ? What has probably happened, and how 

 many years ago, when the knot ends abruptly and is covered 

 on the outer end by the new wood-layers ? How does " Bird's- 

 eye Maple " gets its name, and what are the " eyes " in its 

 wood ? Why is a knot usually much harder than the rest of 

 the wood, and why does it fall out readily from a board ? 



212. Cambium. The chief function of the cambium is 

 to produce new tissues. The cells of the cambium-layer are 

 living and capable of active growth and division, the new 

 cells formed being mostly converted into new " water-tubes," 

 new " food-tubes," and new fibres, while some remain un- 



