Nature and development of plants . cam Fig. 53. Cross-section of a stem of castor bean in which the formationof the cambium zone, cam, as a ring of regular cells, has been completed;p, pith; v, vascular bundles; st, stereome; c, cortex. Compare Fig. 39.—H. O. Hanson. THE CAMBIUM ZNE from the cambium. While the vascular bundles are verj smallit will be noticed that the parenchyma cells separating the vascu-lar bundles begin to divide so as to form a line of cells connectingthe cambium of each bundle (Fig. 52). In some cases thesedivisions are at firsl somewhal irregular but soon the growthresul
Nature and development of plants . cam Fig. 53. Cross-section of a stem of castor bean in which the formationof the cambium zone, cam, as a ring of regular cells, has been completed;p, pith; v, vascular bundles; st, stereome; c, cortex. Compare Fig. 39.—H. O. Hanson. THE CAMBIUM ZNE from the cambium. While the vascular bundles are verj smallit will be noticed that the parenchyma cells separating the vascu-lar bundles begin to divide so as to form a line of cells connectingthe cambium of each bundle (Fig. 52). In some cases thesedivisions are at firsl somewhal irregular but soon the growthresults in the formation of cells with parallel walls and in thisway a ring of regular cells, the cambium zone, i> formed whichcontinues t divide as already noted in the cambium of the bundles (Fig. 53). Consequently new cells are now added notalone to the vascular bundles but also along the entire extent. Fig 54 1 it i item castor bean three weeks older than one Bhown in Fig. 53. Note the changes thai have occurred in the stemand especially the numerous cells thai have been added t the ; /•//. phloem; cam, cambium zone. II. 0. Hanson of the cambium /one. This growth results in the formationof a layer of xylem on the inside of the cambium zone and a layer Of phloem on tin- OUtside, and so brings about the principal enlargement of die stem (Fig. 54). This growth is rep NATURE OF PLANTS 77 each spring in such plants as continue to enlarge from year toyear, as in the shrubs and trees. The great majority of all thecells that are to be found in a years growth are cut off from thecambium zone early in the spring, the first cells, and in fact themajority of all cells formed, being xylem cells and later a fewcells are added to the phloem. Consequently the xylem increasesfaster than the phloem and forms the bulk of the tissues of thestem. The amount of the xylem added to the stem each
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