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 also stores 

 considerable food in a form suitable for the use of the stem. 



As growth proceeds, the cambium layer constantly grows outward, and 

 each new year new fibrovascular bundles are added to supply the new 

 leaves and branches of that season. This accounts for the fact that in 

 cross sections of small twigs some of the medullary rays appear to start 

 from the pith, some from the outer edge of the first annual ring, and still 

 more from each succeeding outer ring. 



The outer bark of the tree is protective. The cells are nearly all dead, 

 and the heavy woody skeletons keep out cold and dryness, as well as pre- 

 vent the evaporation of the fluids within. Most trees are pro\nded with a 



Fibrovascular bundle of a dicotyledon; 

 ph, region of sieve tubes; c, cam- 

 bium; d, duct; and /. fibers of the 

 woody part of the bundle. 



