. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. GROWTH OF VASCULAR BUNDLES 53 the tropics on the approach of winter, and when these plants again clothe themselves with leaves a ring of growth is formed as already described. Such facts as these strengthen the conception that the formation of the ring of growth is at first stimulated by the demand for water on the part of the leaves. Secondary increase in thickness in roots does not differ essentially from that of stems, and the slight differ- ence that occurs is
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. GROWTH OF VASCULAR BUNDLES 53 the tropics on the approach of winter, and when these plants again clothe themselves with leaves a ring of growth is formed as already described. Such facts as these strengthen the conception that the formation of the ring of growth is at first stimulated by the demand for water on the part of the leaves. Secondary increase in thickness in roots does not differ essentially from that of stems, and the slight differ- ence that occurs is due to the pecu- liar arrangement of the phloem and xylem in the root bundle. It will be remembered that the phloem and xylem strands in roots stand side by side and not in radial line as in stems. (Compare diagrams in Fig. 21.) When secondary increase in thickness begins, the cambium flank- ing the phloem on the inside or toward the center lays down xylem elements, so that, with the already •existing phloem, a collateral bundle, such as is typical in stems, is pro- duced. At the same time the cam- bium in front of the original or primary xylem commonly forms a medullary ray (Fig. 26), but it some- times makes phloem elements and thus completes a collateral bundle here also. The cambium then con- tinues to add new phloem and new xylem in both cases, and secondary medullary rays as the dimensions of the xylem and phloem wedges increase, and the root soon comes to look quite like a stem, the discernible difference being. Fig. 26.—Cross section of a young root of Phaseolus multi- florus. A, pr, cortex; m, pith; x, stele (all tissues within the endo- dermis collectively); g, g, g, g, primary xylem bundle; b, b, b, b, primary phloem bundle: the cam- bium, not indicated here, has the same location as indicated in C and D, Fig. 21. B, cross section through older portion of root of the same plant. 6', b\ secondary bast; fe, fe, periderm. The remain- ing letters stand for the same t
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