. A compendium of general botany. Plants. 90 COMPENDIUM OF GENERAL BOTANY. supposed to be due to an increased cortical pressure in the fall of the year (Sachs, de Yeies). This assumption can no longer be maintained in the light of present knowledge of physiological science.' Even if we were concerned only with apparent transfor- mations due to pressure, namely, tangential spreading, or radial flat- tening, this hypothesis would not be tenable, because of tlie fact that radial pressure, in the fall, increases only slightly or may even di- minish. However that may be, it is evident that the fina
. A compendium of general botany. Plants. 90 COMPENDIUM OF GENERAL BOTANY. supposed to be due to an increased cortical pressure in the fall of the year (Sachs, de Yeies). This assumption can no longer be maintained in the light of present knowledge of physiological science.' Even if we were concerned only with apparent transfor- mations due to pressure, namely, tangential spreading, or radial flat- tening, this hypothesis would not be tenable, because of tlie fact that radial pressure, in the fall, increases only slightly or may even di- minish. However that may be, it is evident that the final decision as to whether a given cambium-cell will develop into libriform, wood-parenchyma cell, or a vessel with its characteristic thickenings of the walls, cannot simjjly depend upon a greater or lesser pressure exerted by the cortex. Krabbe in his first publication referring. Fig. 55.—Medullary rays of Cystisus Laburnum. A, Large and Cm, small medullary rays in tangential longitudinal section; F, ray in radial longitudinal section. (After Haberlandt.) to this subject questions rightly: "Is the vessel perhaps a large libriform-cell, or is it, vice versa., a small vessel?" The annual rings are not equally distinct in the different trees and shrubs. A radial longitudinal section of dicotyledonous or coniferous wood in surface-view must of necessity show, scattered here and there, the medullary rays whose elements always cross the longitu- dinally extending elements at riglit angles. I can only point out the fact that cells bearing a resemblance to tracheids form the medullary rays of some conifers. The length of the medullary rays, their size and the number of cells in transverse direction, the origin ' Compare Krabbe, Uber die Bezieliungeu der Rindenspaunung, etc., Sltz- ungsber. der Berl. Akad., 1883. Also Krabbe, IJber das Wachstbum des Ver- dickuugsringes, etc., Abbaudl. der Berl. Akad., Please note that these images are extracted from scanned page image
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectplants, bookyear1896
