NATURE OF PLANTS 77 



also of common occurrence in the phloem (Fig. 43, st). The 

 cambium is composed of very delicate cells, owing to the fact 

 that in this region cell division is taking place and the cells 

 consequently have not yet attained their characteristic form (Fig. 

 42), c). We may best gain an idea of the growth of the cambium 

 if we consider it as a single layer of cells between the xylem and 

 phloem. Each of these cells divides into two daughter cells 

 (Fig. 44), usually the inner cell {i. e., the cell next to the xylem) 

 develops into a duct, tracheid, or other element of the xylem, 

 while the outer cell, after increasing to the original size of the 

 cambium cells, divides into two cells as at first. This process 

 is repeated again and again, usually the inner cell growing into 

 one of the cells of the xylem while the outer cell retains the 



Fig. 44. Diagram showing the mode of division of the cambium cells. 

 The cambium cell is shaded to distinguish it from the cells derived from it. 

 Note in the last division at the right that the inner daughter cell becomes 

 the cambium cell while the outer develops into a phloem cell. 



power of further division. Less commonly the reverse method 

 of growth takes place and we have the outer cell enlarging and 

 forming ope of the cells of the phloem while the inner cell acts 

 as a cambium cell. In this way new cells are added to the xylem 

 and phloem and the enlargement of the vascular bundle is 

 effected. 



35. The Conducting System of the Plant. — These vascular 

 bundles extend from the root up through the stem and branches 

 to the leaf where they divide again and again, reaching all parts 

 of it. In this way the bundles become much reduced in size, 

 the free end of the vein often consisting of a single tracheid to 

 conduct the crude materials to the surrounding cells and elon- 

 gated parenchyma cells to collect the manufactured foods (Fig. 

 5, I). We now see how admirably the vascular bundles are 

 adapted to the transport of fluids. The xylem carries the sub- 



