14 



AMERICAN MUSEUM GUIDE LEAFLETS 



<~-Oney( 



irear 

 old 



cambium produces new wood everywhere on its inner surface (living 



material while forming, non-living when formed), a cylinder of new 

 wood enfolding immediately the wood of the year before (Fig. 7). Wood 



never lengthens after once formed, so a 

 trunk grows in height only by additions 

 from living buds at the top i Fig. 8), bul 

 a tree grows in diameter annually by jusl 

 the thickness of the new cylinder of wood 

 (a ring of wood when seen at the v\\A\ . 'The 

 thickness of the annual layer in any species 

 depends on the length of the growing season 

 in the given region and on the age of the 

 tree, those wood cylinders formed in early 

 life being relatively tliiek-walled and later 

 ones successively more and more thin-walled. 

 A further fact is true, however, and to be 

 remembered in economic tree planting, that 

 the diameter increase of any tree is always 

 immediately dependent on its growing 

 space, on soil, light and other conditions of 

 the surroundings, factors which to a large 

 extent we can control. 



This method of growth places the older 



wood as "heartwood" in the center of the 



trunk, while the younger wood, called 



"sapwood," is outside of this. Heartwood 



and sapwood may differ in weight and in 



color, since the heartwood is likely to he 



a storehouse for coloring matter, or gummy, 



resinous or mineral substances which have 



come into the tree from the soil or are the 



waste from vital processes. Heartwood is 



more durable than sapwoo I because it does 



not often contain starch or other organic 



f,g. 7. wh.te ash tw,g matter > and therefore is less liahle to the 



Cioss section to show rings of attacks of insects and to the growth of the 



weod, corresponding to the cylin- organisms thai cause decay, also on its 



ders of wood in the twig. Com- change from sapwood depositions of 



P are with V] - 8 mat rial more or less antiseptic take place. 



Trees in which the formation of heartw 1 does not follow rapidly on the 



growth of sapwood are the oaks, elms, walnuts and pines. Some trees 



.Jl 



wo uears 





-Tliree years 

 old 



