40 ECONOMIC WOODS OF THE UNITED STATES 



Von Hohnel, Franz Ritter: Ueber den etagenformigen Aufbau einiger 

 Holzkorper, Berichte d. deutschen Bot. Gesellschaft, Vol. II, Berlin, 

 1884, pp. 2-5. 



GROWTH RINGS 



A tree increases in diameter by the formation between the 

 old wood and the inner bark of new woody layers which envelop 

 the entire stem and living branches. In cross section, as on the 

 end of a log, these layers appear as concentric zones or rings 

 (Fig. 1). The distinction between contiguous rings is due to 

 structural peculiarities, augmented in some instances by local 

 deposit of resin or pigment. Each ring consists of two more or 

 less readily distinguishable parts, the inner, called early wood 

 (spring wood), and the outer, or late wood (summer or autumn 

 wood). 



In ring-porous woods (Frontispiece; Plate III), such as Quercus, 

 Castanea, Fraxinus, and Robinia, the larger vessels become local- 

 ized in the early wood, thus forming a region of more or less open 

 and porous tissue, while the wood fibres preponderate in the late 

 wood, thereby producing a much denser layer. In other instances, 

 as in Acer, Magnolia, JEsculus, and Liquidambar (Plate VI), where 

 the vessels are fairly uniformly distributed — diffuse-porous — the 

 occurrence of growth rings may be due to one or more of the 

 following conditions: (1) a gradual diminution in size of the 

 vessels toward the periphery of the ring; (2) a decided reduction 

 in number of the vessels in the late wood; (3) a change in kind of 

 the wood elements, e.g., where the outer layer of late wood consists 

 wholly or chiefly of wood parenchyma or of tracheids; (4) increase 

 in thickness of the wall of the wood elements near the limit of the 

 late wood. 



In Gymnosperms where vessels are wholly absent growth rings 

 are due to variations in the tracheids. Viewed in cross section 

 the cells of early wood are relatively large, thin-walled, and more 

 loosely aggregated; while those of the late wood are smaller, 

 thicker-walled, closely packed together and very often radially 

 flattened, presumably as a result of cortical pressure (Fig. 8; 

 Plate II, Figs. 1, 2, 4). This transition from open to dense structure 

 may be gradual, as in the soft pines, or very abrupt, as in many 

 hard pines. Not infrequently the dense aggregation of cells 

 involves a deepening of the color peculiar to the tissue as a whole. 

 In any wood it is almost invariably the apposition of the more open 



