HOW TREES GROW. 



273 



especially pines, by the dark colored summer wood; while in maple, birch, tulip, etc., only a thin 

 Hue of flattened, hence darker and regularly aligned, sunnier cells, often hardly rrcogni/ahle. 

 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 (fig. 36). We can, in fact, retrace the history of its growth, the, vicissi- 

 tudes 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 cessation of growth. 

 Only exceptionally a tree may fail to make its 

 growth throughout the whole length, on account 

 of loss of foliage and other causes, and occasion- 

 ally, when its growth has been disturbed during 

 the season, a "secondary" ring, resembling the 

 annual ring, and distinguishable only by the ex- 

 pert, may appear and mar the record. 



To the forest planter this chapter on ring- 

 growth is of great importance, 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 therefrom deter- 

 mining when it is most profitable for him to har- 

 vest (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 the pre- 

 ponderance 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. 



FIG. 34. Trees in and out of the forest. D, tree grown in the 

 open; D', tree grown in the forest. 



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 (figs. 37 aud 38). 



There are, to be sure, different natural types of development, some, like the walnuts, oaks, 

 beeches, and the broad leafed trees generally, having greater tendency to spread than others. like 

 spruces, firs, and conifers in general, which lengthen their shaft in preference to spreading, even 

 in the open. This tendency to spreading is also influenced by soil conditions and climate, as well 

 as by the age of the tree. When the trees cease to grow in height their crowns broaden, and this 

 takes place sooner in shallow soils than in deep, moist ones; but the tendency can be checked and 

 all can be made to develop the shaft at the expense of the branches by proper shading from the 

 sides. 



H. Doc. 181 18 



