142 lUSTOLOGY 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 Hne 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. 

 Bi) are deposits of tannin; in quassia wood, starch; in caneUa 

 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, hke 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 ouF knowledge concerning the function of latex in some 



