84 PROPERTIES OF WOOD. 



of a tree. At an advanced age the annual zones become 

 gradually narrower, so that the annual increment may remain 

 constant for a number of years. Mayr has shown that, as a 

 tree becomes older and its volume of wood increases, it 

 becomes less sensitive to variations in daily and even annual 

 variations of atmospheric temperature and humidity. The 

 large volume of wood then equalizes extremes of tempera- 

 ture, so that the cambium being equably nourished produces 

 a steadier increment and consequently wood that is more finely 

 grained than in young trees. 



The method of growing a tree, and supplying it with light 

 and heat is also important. Virgin forests yield wood that 

 may be more knotty than wood from a dense artificial crop, 

 but on clean boles exhibits the finest grain, the greatest 

 uniformity of structure (Fig. 39). Excluding very old trees, 



Fig, 3!>. Wood from a virgin forest. 



the explanation of this fact is due to the prolonged mainten- 

 ance of the tree during youth in the diffused shade of the 

 virgin forest ; for decades the young plant has lived protected 

 by older trees under uniform conditions of temperature, 

 humidity and illumination, the forest equalising extremes. 

 By the successive death of old neighbours, the tree obtains 

 gradually a full supply of light and heat long after the 

 passing of its youth, when meteorological extremes would 

 have caused irregularity in its annual woody zones. 



The gradual acquisition of an open position results in an 

 increased increment, but not in abnormally wide and irregular 

 annual zones. Wood from a Selection forest approaches 

 this condition most nearly. 



A tree grown under the Shelterwood compartment system, 

 with a dense reserve of mother-trees (Fig. 40) possesses near 

 its pith, wood, that has very narrow zones for the first 20 or 

 40 years ; then, owing to full exposure to light, a number of 



