142 HISTOLOGY OF MEDICINAL PLANTS 
Structure of Cells 
The structure of the individual cells forming the medullary 
rays differs greatly in different plants, but is more or less con- 
stant in structure in a given species. 
The medullary rays of the wood. usually have strongly pitted 
side and end walls, while the medullary rays of most barks are 
not at all, or only slightly, pitted. In most plants the cells are 
of nearly uniform size. Frequently, however, the cells vary in 
size in a given ray, as shown in the cross-section of kava-kava. 
Arrangement of the Cells in a Ray 
The union of any two cells in a ray is also of importance. 
In quassia the medullary ray cells have oblique end walls, so 
that on cross-section the line of union between two cells is an 
oblique wall. In most plants the medullary ray cells have 
blunt or square or oblique end walls, so that the line of union 
is a straight line. 
In most plants the cells are much longer than broad, but the 
cells of sassafras bark are nearly as broad as long. 
The walls of the cortical medullary ray cells and the medul- 
lary rays of most roots and stems of herbs are composed of cellu- 
lose; while the walls of medullary ray cells occurring in woods 
are frequently lignified. 
There is a great variation in the character of the cell con- 
tents of medullary rays. In white pine bark (Plate 48, Fig. 
Br) are deposits of tannin; in quassia wood, starch; in canella 
alba, rosette crystals of calcium oxalate, etc. 
LATEX TUBES 
Living latex tubes, like sieve tubes, have a layer of proto- 
plasm lining the walls, and, in addition, have numerous nuclei. 
In drug plants the nuclei are not distinguishable, but the proto- 
plasm is always clearly discernible. 
Latex tubes function both as storage and as conducting cells. 
They, like the sieve tubes, contain proteid substances chiefly, 
yet frequently starch is found. The cells bordering the latex 
tubes absorb from them, as needed, the soluble food material. 
While our knowledge concerning the function of latex in some 
——— 
