268 TIMBERS AND THEIE USES 



very heavy. It is hardly surprising then to 

 learn that with the opening of the leaves the 

 cells of the wood tissue which are formed are 

 thin walled and of large diameter. At leaf fall, 

 the cells of the same tissue are produced with 

 thick walls and of small diameter. It is this 

 alternate formation of wide and narrow cells 

 which gives the appearance of rings on a cross 

 section. The autumn formed wood is so close 

 that, without the aid of a pocket lens, the 

 cavities or pores cannot be distinguished ; in 

 the depth of winter the formation of wood 

 practically ceases ; with the advent of spring, 

 the large diameter tissue is formed ; so large are 

 the cells of this tissue that they may be plainly 

 seen with the naked eye in most woods. As 

 the change from the non-porous to the porous 

 tissue is sudden, it imparts to the cross section 

 of the wood the appearance of a ring. Normally 

 a single ring is formed in each year, so that the 

 annual rings afiord a guide to the age of the 

 tree. Occasionally more than one ring may be 

 formed in a year, and this is the case when the 

 tree is completely defoliated by insects and, 

 later in the year, sends forth fresh leaves. 

 Frosts binding the tree tightly and other 

 external and unusual conditions may disturb 

 the regular sequence of the autumn and spring 

 wood. 



