WRIGHT : THE GENUS DIOSPYROS 
34 
of double rays. In tracing one ray from the younger to the 
older wood it was first seen to be three cells broad, and very 
gradually one marginal row was replaced by a row of fibres, 
and finally, the double ray passed into a single ray, each 
component having a tangential diameter equal to the sum of 
those forming the double ray. It has also been noticed in 
specimens of D. attenuata, that whenever the ray is more 
than one cell broad the component cells are invariably of 
the horizontal type, and at each end of such a ray the 
components are of the vertical type with large tangential 
diameters. 
It is to be noted that whenever the ray is more than one 
cell broad the components have always a small tangential and 
large radial diameter, and many sections show a double 
ray superposed on a broad single ray, each ray extending 
vertically through a distance of many cells. 
The medullary rays form large vertical strands of tissue 
from twelve to over thirty cells in height, the components 
having very variable dimensions and forming part of single, 
double, and triple rays. 
The medullary rays, since they pass through all the 
secondary xylem, come into direct communication with every 
other type of cells, but the most striking communication is 
with the wood parenchyma, which they so closely resemble 
(particularly the vertical type) in structure, thin walls and 
crowded contents. 
Form and size can be treated better in a combined descrip¬ 
tion of the differentiation of medullary ray tissue. 
In nearly every species there is a well marked differentia¬ 
tion into (a) horizontal cells characterized by large radial 
and short tangential and vertical diameters, and ( b) vertical 
cells having short radial and large tangential and vertical 
diameters. In many cases one feels strongly inclined to 
introduce yet another type having approximately the form 
of a cube. 
