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axis-cylinder (see Fig. 5), and especially by means of tangential walls. The result of this activity is the development of a cambium layer, as it is called, immediately inside the five phloëm groups of the axis-cylinder, and this layer becomes continuous all round the axis-cylinder, but is so arranged that it runs outside the primary xylem groups and inside the primary phloëm groups (Fig. 24, cam). This cambium layer is a hollow cylindrical layer of thin-walled cells, full of protoplasm, and somewhat longer than they are broad or deep, and these cells have the peculiarity of dividing very rapidly, especially by tangential walls, so that cell multiplication goes on very rapidly, and the layer would soon become very thick if no other changes occurred. As the new cells are formed, however, those on the outer side of the cylinder—i.e., those nearest the phloëm—become for the most part converted into sieve-tubes and cells of the phloëm; while the much more numerous cells formed on the inner side—i.e., nearest the center of the axis-cylinder—are chiefly converted into vessels and cells of the xylem. This xylem and phloëm developed by the cambium are termed secondary xylem and secondary phloëm respectively, and it will be noticed that whereas the secondary phloëm is deposited radially on the inner side of the primary phloëm, the secondary xylem is placed between the primary xylem groups, and not radially outside them (Fig. 24, se.x and Moreover, the youngest vessels are now nearest the cambium, whence the order of development has become the con-