THE STRUCTURE OF WOOD. 21 



It is only at the tips of the branches that the cambium cells grow 

 much in length; so that if a nail were driven into a tree twenty 

 years old at, say, four feet from the ground, it would still be four 

 feet from the ground one hundred years later. 



Looking once more at the cross-section, say, of spruce, the inner 

 portion of each ring is lighter in color and softer in texture than 

 the outer portion. On a radial or tangential section, one's finger 

 nail can easily indent the inner portion of the ring, tho the outer 

 dark part of the ring may be very hard. The inner, light, soft 

 portion of the ring is the 

 part that grows in the 

 spring and early summer, 

 and is called the "spring 



Fig-. 12.Diagram Showing- the Modeof Div i- 



wood" while the part that sion of lh . e ^ a bium . c * Us - . ? cambium 



cell is shaded to distinguish it from the 



crrnwQ lafpr in flip 5Pa<?on i^ cells derived from it. Note in the last di- 



vision at the right that ihe inner daughter 



ppllprl "sjnmmpr wnnrl " A<? ce ll becomes the cambium cell while the 



WOOd. AS outer cell developg into a bast celh From 



the Summer WOOd is hard Curtis: Nature and Development of Plants. 



and heavy, it largely deter- 

 mines the strength and weight of the wood, so that as a rule, the 

 greater the proportion of the summer growth, the better the wood. 

 This can be controlled to some extent by proper forestry methods, as 

 is done in European larch forests, by "underplanting" them with 

 beech. 



In a normal tree, the summer growth forms a greater proportion 

 of the wood formed during the period of thriftiest growth, so that 

 in neither youth nor old age, is there so great a proportion of sum- 

 mer wood as in middle age. 



It will help to make clear the general structure of wood if one 

 imagines the trunk of a tree to consist of a bundle of rubber tubes 

 crushed together, so that they assume angular shapes and have no 

 spaces between them. If the tubes are laid in concentric layers, first 

 a layer which has thin walls, then successive layers having thicker 

 and thicker walls, then suddenly a layer of thin-walled tubes and in- 

 creasing again to thick-walled ones and so on, such an arrangement 

 would represent the successive annual "rings" of conifers. 



The medullary rays. While most of the elements in wood run 

 longitudinally in the log, it is also to be noted that running at right 

 angles to these and radially to the log, are other groups of cells 

 called pith rays or medullary rays (Latin, medulla, which means 



