. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. UNUSUAL GROWTH IN THICKNESS 57. Another method of growth in thickness is found in the arbor- escent Liliaceae, represented by the genera Draceena, Yucca, and Aloe. Here, either close to the growing apex, or remote from it in the region of the permanent tissues, a secondary rneristem is formed by the tangential division of the cells of the pericycle, which adds new cells both inward and out- ward. A part of the cells added toward the inside be- come differentiated i
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. UNUSUAL GROWTH IN THICKNESS 57. Another method of growth in thickness is found in the arbor- escent Liliaceae, represented by the genera Draceena, Yucca, and Aloe. Here, either close to the growing apex, or remote from it in the region of the permanent tissues, a secondary rneristem is formed by the tangential division of the cells of the pericycle, which adds new cells both inward and out- ward. A part of the cells added toward the inside be- come differentiated into new vascular bundles (Fig. 29), and a part into new ground meristem; while those, much less in number, formed to- ward the outside constitute a secondary cortex. In this way growth in thickness goes on from year to year, and hafe been known to produce in Dracasna a stem diameter of fifteen feet. In this instance, however, the tree was esti- mated to be six thousand years old. Unusual Growth in Thickness.—Variations from the usual modes of secondary thickening take place in several families of Dicotyledons (Apocynaceae, Sapindaceae, Bignoniaceae, Cheno- podiaceae, Amarantaceae, Phytolaccaceae, Papilionaceae, Nyc- taginaceae), and, under the Gymnosperms, the Cycadaceae and species of Gnetum. A type of frequent occurrence is where the cambium ring soon ceases its activity and a new ring of secondary cambium is formed by tangential division of cells of the pericycle, and after this has laid down a zone of vascular bundles surrounding those first formed it becomes inactive and a new cambium ring giving rise to a new zone of bundles is Fig. 28.—Photomicrograph of cross section of cornstalk somewhat older than in Fig. 27. Compare with Fig. 27, and notice that the number of vascular bundles is approximately the same in both, and the number of cells in the fundamental tissue is approximately the same. Growth in Fig. 28 has been accomplished by the enlarge- ment of the cells alre
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