THE CASTOR BEAN 



109 



FIG. 49. DIAGRAM SHOWING THE METHOD 

 BY WHICH THE CAMBIUM LAYER PRODUCES 

 WOOD CELLS ON ITS INSIDE AND BARK 

 CELLS ON THE OUTSIDE 



be, the cells of the bark; 

 c, cambium cells; 

 we, the wood cells. 



increase in size. As the cells of the cambium layer divide, 

 new cells are formed between the bark and the wood of the old 

 bundles. Some of these new cells are formed inside of the 

 cambium layer, and outside of the xylem, as shown diagram - 

 matically in Figure 49. 

 Other cells are formed 

 on the outside of the 

 cambium and inside 



C 



of the old phloem 

 layer. These new 

 cells soon assume the 

 form of new wood 

 cells, new tracheids or 

 ducts on the inside; 

 while those outside 

 the cambium assume 



the form of sieve cells, parenchyma, etc. It thus comes about 

 that the plant is producing new wood cells in the form of a 

 layer outside the old wood ring, and new phloem cells in a 

 layer inside the old phloem ring. The wood grows by addi- 

 tions upon its outer surface and the bark by additions to its 

 inner surface. Since the cambium forms a complete ring, 

 this method of growth evidently will produce a complete ring 

 of wood around the stem, and since the cambium cells con- 

 tinue to produce new cells during the whole of their active 

 life, they will continue to add new layers of wood on the out- 

 side of the old wood. The wood ring, which at first is only 

 a thin layer just inside the cambium, becomes thicker and 

 thicker as the growth continues. As it becomes thicker, the 

 stem, of course, increases in diameter, and, since the cambium 

 always remains on the outside of the wood, the stem may keep 

 increasing in size as long as the cambium cells are able to de- 

 velop new cells to be deposited as wood cells on the outside 

 of the old wood. In the same way the cambium deposits 

 masses of cells on the inner side of the phloem of the bundles, 



