SECONDARY XYLEM 



067 



strengthened by transverse bars, and bear bordered pits. These cells 

 occur more particularly along the margins of the medullary rays, and 

 serve for the conduction of water in the radial direction. In the 

 medullary rays of Dicotyledons, the central rows of cells are the most 

 typical; they are radially elongated, and serve principally for the 

 translocation and storage of non-nitrogenous plastic material. These 

 cells constitute the so-called procumbent cells of the medullary rays. 

 The elements occupying the upper and lower margins of the rays, on the 



me 

 ml' 



me 



L^i 



Secondary medullary ray of riaus Pumilio (at the beginning of July), me, albu- 

 minous cells ; nit, tracheidal cells ; ml-ml', row of conducting parenchyma cells ; sr, 

 sieve-tubes ; c, cambium ; ft, fibrous trachcides. 



other hand, are often comparatively short and high, and may be dis- 

 tinguished as the upright cells ; they bring the medullary ray into 

 relation with the neighbouring vessels, which communicate with them 

 by means of one-sided bordered pits. 



The so-called medullary spots, defined by De Bary as local 

 hypertrophies of the medullary rays, are of regular occurrence in the 

 wood of the genera Abuts and Sorbus; they are composed of irregularly 

 polyhedral cells, with pitted walls, and appear in the first instance to 

 constitute a form of storage-tissue. This view finds support in the fact 

 that the contents of the cells in question consist very largely of starch 

 and tannin. It is another question, to what extent such medullary 

 spots may act as secretory or excretory organs, or may represent 



