INTRODUCTION 15 



(iv) The medullary rays (vide fig. 67) are composed of 

 two kinds of cells. Firstly, the medullary ray parenchyma 

 cells with thickened cellulose, or shghtly lignified, walls 

 with 2-5 (generally 4) slit-shaped simple pits into each 

 tracheide they cross. These cells contain protoplasm, 

 nuclei, starch, and resin. And, secondly, medullary ray 

 tracheides which are not present in all rays, but are present 

 on the upper and lower sides of many of them, and occur 

 as intermediate layers of some. These cells have thin 

 lignified ^valls with small bordered pits, and contain only 

 water. 



Medulla. This is the star-shaped central portion, having 

 usually five unequal points which are left by the tracks of 

 leaf traces, and thus correspond to the five rows of leaves. 

 The medullary cells have at first cellulose walls, but when 

 secondary wood is formed they l)ecome lignified, thougli 

 the cells may still contain starcli. 



