104 HISTOLOGY OF MEDICINAL PLANTS 
The walls of bast fibres are composed of cellulose or of lignin. 
Most of the bast fibres occurring in the medicinal plants give 
a strong lignin reaction. 
WOOD FIBRES 
Wood fibres always occur in cross-sections associated with 
vessels and wood parenchyma, from which they are distin- 
guished by their thicker walls, smaller diameter, and by the 
nature of the pores, which are usually oblique and fewer in 
number than the pores in the walls of wood parenchyma, and 
different in form from the pores of vessels. 
The wood fibre on cross-section (Plate 105, Fig. 4) shows 
an angled outline, except in the case of the fibres bordering the 
pith-parenchyma, etc., in which case they are rounded on their 
outer surface, but angled at the points in contact with other 
fibres. The pore of wood fibres is one of the main characteristics 
which enable one to distinguish the wood fibres from bast fibres. 
The pores are slanting or strongly oblique (Plate 28, Fig. 2), 
and they show for their entire length on the broadest part of 
the wall—i.e., the upper or the lower surface—while in the side 
wall they are oblique; but they are not so distinct as they are 
on the broad part of the wall. 
Frequently the pores appear crossed when the upper and 
the lower wall are in focus, because the pores are spirally ar- 
ranged, and the pore on the under wall throws a shadow across 
the pore on the upper wall, or vice versa. 
Wood fibres always occur in a broken condition (Plate 28, 
Fig. 1) in powdered drugs. These broken fibres usually occur 
both singly and in groups in a given powder. 
The color of wood fibres varies greatly in the different me- 
dicinal woods. Fragments of wood are usually adhering to 
witch-hazel, black haw, and other medicinal barks. In each of 
these cases the wood fibres are nearly colorless. In barberry 
bark adhering fragments of wood and the individual fibres are 
greenish-yellow. The wood fibres of santalum album are whitish- 
brown; of quassia, whitish-yellow; of logwood and santalum 
rubum, red. 
Some wood fibres function as storage cells. In quassia the 
