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FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGK1CULTUKE. 



GROWTH IN THICKNESS. 



The young seedling and the young shoot of the older tree much resemble in interior structure 

 thsit of any herbaceous plant, being composed of a large amount of pith, loose squarish cells, and 

 a few bundles of loug libers symmetrically distributed about the center, the whole covered with a 

 thin skin or epidermis. Each strand or bundle of fibers, called fibro- vascular (fiber vessel) bundles, 

 consists of two kinds, namely, wood fibers on the inner side and bast fibers of different structure 

 on the outer side. Between these two sets of fibers, the bast aud the wood, there is a row of cells 

 which form the really active, growing part of the plantlet, the cambium. The cambium cells are 

 actively subdividing and expanding, giving oft' wood cells to the interior and bast cells to the 

 exterior, and extending at the same time sidewise, until at the end of the season not only are 



the wood and bast portions increased in lines radiating 

 from the center, but the cambium layer, the wood cells, 

 and the bast cells of all the bundles (scattered at the 

 beginning) join at the sides to form a complete ring, or 

 rather cylinder, around the central pith. Only here aud 

 there the pith cells remain, interrupting the wood cylinder 

 and giving rise to the system of cells known as medullary 

 rays. The cross section now shows a comparatively small 

 amount of pith aud bast or bark and a larger body of 

 strong wood fibers. The new shoot at the end, to be sure, 

 has the same appearance and arrangement as the young 

 plantlet had, the pith preponderating, and the continuous 

 cylinder of cambium, bast, and wood being separated into 

 strands or bundles. 



During the season, through the activity of the cambial 

 part of the bundles, the same changes-take place in I lie 

 new shoot as did the previous year in the young seedling, 

 while at the same time the cambium in the yearling part 

 also actively subdivides, forming new wood and bast cells, 

 and thus a second ring, or rather cylinder, is formed. The 

 cambium of the young shoot is always a continuation of 

 that of the ring or cylinder formed the year before, aud 

 this cambium cylinder always keeps moving outward, so 

 that at the end of the season, when activity ceases, it is 

 always the last minute layer of cells on the outside of the 

 wood, between wood proper and bark. It is here, there- 

 fore, that the life of the tree lies, and any injury to the 

 cambium must interfere with the growth and life of the 

 tree. 



The first wood cells which the cambium forms in the 

 spring are usually or always of a more open structure, 

 thin-walled, and with a large opening or "lumen" com- 

 parable to a blown-up paper bag; so large, in fact, sometimes, is the "lumen" that the width of 

 the cells can be seen on a cross section with the naked eye, as, for instance, in oak. ash, elm, the 

 so-called "pores" are this open wood formed in spring. The cells which are formed later in 

 summer have mostly thick walls, are closely crowded and compressed, and show a very small 

 opening or "lumen," being comparable, perhaps, to a very thick, wooden box. They appear in 

 the cross section not only denser but of a deeper color, on account of their crowded, compressed 

 condition and thicker walls. Since at the beginning of the next season again thin-walled cells 

 with wide openings or lumina are formed, this difference in the appearance of "spring wood" 

 and "summer wood" enables us to distinguish the layer of wood formed each year. This "annual 

 ring" is more conspicuous in some kinds than in others. In the so-called "ring-porous" woods, 

 like oak, ash, elm, the rings are easily distinguished by the open spring wood; in the conifers, 



FIG. 33. Development in and out of the forest. A, 

 young tree alike in both cases ; J? and C, successive 

 stages of tree grown in the open ; Ji' and C", corre- 

 sponding stages of tlie tree grown in the forest. 

 Numbers refer to annual growth in height. 



