FORESTRY AND CONSERVATION 337 



loose squarish cells, and a few bundles of long fibers symmetrically 

 distributed about the center, the whole covered with a thin skin or 

 epidermis. Each strand or bundle of fibers, called fibre-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 and 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 off 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 begin- 

 ning) join at the sides to form a complete ring, or rather hollow 

 cylinder, around the central pith. Only here and 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 and 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 the 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, 

 and 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, therefore, 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," comparable 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 com- 

 pressed, and show a very small opening or "lumen," being compara- 

 ble, 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 be- 

 ginning of the next season again thin-walled cells with wide open- 

 ings or lumina are formed, this difference in the appearance of 



