270 THE STlitJCTTTEE OF TIMBEK. 



anatomical peculiarities of the different varieties of timber, but 

 rather to describe in plain language how a bit of wood has been 

 produced, and to call attention to some of its more prominent 

 features. 



Wood is a product of the growth of a tree in two directions, 

 namely, in height and in thickness. The height-growth of a 

 tree is entirely confined to that portion of the stem which is re- 

 presented by the youngest annual shoot. Further back in the 

 stem the tree is undergoing no vertical extension, and the tale 

 that is sometimes told us of the boy being unable to pass erect 

 beneath a certain branch, while fifty years afterwards the man 

 has no difficulty in so doing, may be explained by the surface of 

 the ground having been worn down or depressed, but not by the 

 base of the branch having been elevated. 



During the winter there is no growth in height, and all the 

 tissues that will form the height-growth of the tree during the 

 following season of growth are at that time confined within the 

 bud which usually terminates a stem or branch. 



Perhaps the clearest idea of how growth in height really takes 

 place may be obtained by examining a young shoot of a tree in 

 the middle of the growing season, after vertical extension has 

 begun but before it has been completed. For this purpose the 

 shoot may be regarded as divided into three regions or zones. 

 At the extreme apex we have a portion of tissue where the cells 

 are very small, of nearly the same diameter in all directions, 

 possessed of very thin walls which are easily extensible, and 

 full, or nearly full, of that vital substance common to all plants 

 and animals which bears the name of protoplasm. It is in this 

 portion of the shoot that new cells are being formed by the 

 process known as cell-division. A fresh cell-wall cuts across a 

 cell and divides it into two equal portions, and these two new 

 cells may subsequently undergo the same change, so that from 

 one cell we get two, from two, four, from four, eight, and so on. 

 liut it is quite evident that if nothing more than cell-division 

 were going on in a young shoot we might have indefinite multi- 

 plication of new cells without, however, obtaining any absolute 

 increase in the volume or weight of the shoot. We may com- 



