338 HORTICULTURE, FORESTRY, FLORICULTURE 



"spring wood" and "summer wood" enables us to distinguish the 

 layer of wood formed each year. This "annual ring" is more con- 

 spicuous in some kinds than in others. In the so-called "ring por- 

 ous" woods, like oak, ash, elm, the rings are easily distinguished 

 by the open spring wood; in the conifers, especially pines, by the 

 dark-colored summer wood; while in maple, birch, tulip, etc., only 

 a thin line of flattened, hence darker and regularly aligned, sum- 

 mer cells, often hardly recognizable, distinguishes the rings from 

 each other. Cutting through a tree, therefore, we can not only 

 ascertain its age by counting its annual layers in the cross section, 

 but also determine how much wood is formed each year. We can, 

 in fact, retrace the history of its growth, the vicissitudes through 

 which it has passed, by the record preserved in its ring growth. 



To ascertain the age of a tree correctly, however, we must cut 

 so near to the ground as to include the growth of the first year's 

 little plantlet; any section higher up shows as many years too few 

 as it took the tree to reach that height. 



This annual-ring formation is the rule in all countries which 

 have distinct seasons of summer and winter and temporary cessa- 

 tion of growth. Only exceptionally a tree may fail to make its 

 growth throughout its whole length on account of loss of foliage or 

 other causes; and occasionally when its growth has been disturbed 

 during the season, a "secondary" ring, resembling the annual ring, 

 and distinguishable only by the expert, may appear and mar the 

 record. 



To the forest planter this chapter on ring growth is of great im- 

 portance, because not only does this feature of tree life afford the 

 means of watching the progress of his crop, calculating the amount 

 of wood formed, and there-from determining when it is most profit- 

 able for him to harvest (namely, when the annual or periodic wood 

 growth falls below a certain amount), but since the proportion of 

 summer wood and spring wood determines largely the quality of the 

 timber, and since he has it in his power to influence tjie preponder- 

 ance of the one or other by adaptation of species to soils and by their 

 management, ring growth furnishes an index for regulating the 

 quality of his crop. 



Form Development. If a tree is allowed to grow in the open, 

 it has a tendency to branch, and makes a low and spreading crown. 

 In order to lengthen its shaft and to reduce the number of branches 

 it is necessary to narrow its growing space, to shade its sides so that 

 the lower branches and their foliage do not receive light enough to 

 perform their functions. When the side shade is dense enough, 

 these branches die and finally break off under the influence of winds 

 and fungous growth ; wood then forms over the scars and we get a 

 clean shaft which carries a crown high up beyond the reach of 

 shade from neighbors. 



The branches being prevented from spreading out, the shaft is 

 forced to grow upward, and hence ; when crowded by others, trees 

 become taller and more cylindrical in form, while in the open where 

 they can spread, they remain lower and more conical in form. 



