. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. BUDS AND STEMS 109 bundles ultimately to unite along the outer edge because of more rapid growth in that region. This rapidly growing area, which extends com- pletely around the stem under the bark as a hollow cylinder, is called the cambium layer. All growth takes place from that part of the stem. As the bundles squeezed together in their growth, the pith or parenchyma became compressed into thin plates, the edges of which are seen in the cross section. We call these plates the medullary r
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. BUDS AND STEMS 109 bundles ultimately to unite along the outer edge because of more rapid growth in that region. This rapidly growing area, which extends com- pletely around the stem under the bark as a hollow cylinder, is called the cambium layer. All growth takes place from that part of the stem. As the bundles squeezed together in their growth, the pith or parenchyma became compressed into thin plates, the edges of which are seen in the cross section. We call these plates the medullary rays. Microscopic Structure of a Fibrovascular Bundle in the Dicoty- ledonous Stem.—The structure of one of the young bundles in a young dicotyledonous stem is somewhat as seen in the illustration. The bundle is composed of two areas. The inner area, directed toward the middle of the stem, is made up of woody, thick-walled cells, which support and in some cases form part of the walls of the tubes which carry the soil water up the tree. These tubes differ considera- bly in size, the larger ones being formed during the more rapid spring growth of the bundle. The outer part of the bun- dle, which is separated from the inner part by the cambium layer, is quite dif- ferent in structure from the inner part. It is, in fact, growing toward the outside and is forming the inner layer of the bark. The cells of the cambium layer are much softer and have thinner walls than those of the wood, because they are filled with protoplasmic material and are constantly dividing to form new cells. Most of the cells of the inner bark are extremely tough and fibrous. Be- tween the bast fibers, as the tough cells are called, are found numerous elongated cells joined end to end, the ends of each cell being full of little holes. These are the sieve tubes (or soft bast cells); they serve as a channel for the sap or food materials which come down from the leaves toward the roots. This region of the stem al
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