WOODY STEMS 245 
cambium (5) is typical in form, and it has formed one or two 
layers of phelloderm cells (6) which have the same form as the 
cambium cells but with thicker walls. Next to the phelloderm 
occur the cortical parenchyma cells. The remaining structure 
of the mature stem is identical with that of Fig. 2. 
POWDERED BUCHU STEM 
Powdered buchu stem (Plate 102) has many striking features 
which make it easy of identification when mixed with buchu 
leaves. A few unicellular, rough, thick, white-walled trichomes 
(1) occur distributed throughout the field. They are straight 
or slightly curved and vary in length from 40 to 100 microns; 
in thickness at the bast they measure from 10 to 22 microns. 
The central cavity varies greatly, and in some trichomes seems 
to have disappeared entirely. The epidermal cells (2) are very 
characteristic, occurring singly or in groups of two or more. 
The cells from the older stems often appear. reddish brown by 
transmitted light, while the epidermal cells from the younger 
stems appear whitish opaque (porcelain-like). They are usually 
six-sided and angular in outline. The cortical parenchyma 
cells (3) on transverse view have a rounded cell cavity and 
intercellular spaces between the walls. The double walls vary 
in thickness, the greatest thickness being about 9 microns. The 
parenchyma cells (3) on longitudinal view show square ends 
and often contain sphero-crystalline masses of hesperidin. The 
thin-walled sieve cells and the surrounding cells are scarcely 
ever seen in the powder. The white-walled pointed stereomes 
(4) are a characteristic feature of the powder; they vary greatly 
in length, in diameter and in the thickness of their walls. In 
a number eighty powder the fibres are mostly broken. The 
greatest length of the unbroken fibres is 1.25 microns. The 
thickest wall measured 5 microns and the greatest observed 
width was 25 microns. The spiral reticulate and scalariform 
thickened conducting cells occur scattered throughout the 
powder. ‘The reticulate and scalariform cells usually occur with 
wood fibres. It is an interesting fact that the spiral thickening 
in conducting cells is usually separate from the side wall and 
nearly always appears as indicated at 5. An occasional rosette 
crystal of calcium oxalate (6) is seen in the field. The wood 
