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., IBSi. 



