412 DR. P. GROOM AND MR. W. RUSHTON ON THE STRUCTURE 
In addition to ray-tracheids, there are two kinds of parenchyma-cells, 
thick-walled and thin-walled. 
Each thick-walled parenchyma-cell is radially elongated and in length equal 
to the radial diameters of 4-6 tracheids in the spring-wood and about 
8 tracheids in the summer-wood. In tangential view the lumen is an upright 
oval, and the general contour is vertically oblong with rounded angles in 
uniseriate rays, but the contour is more rounded near the central parts of 
the fusiform rays. ‘Their end walls are oblique or transverse. Оп the 
lateral walls the pits are very small, half-bordered (Picea-like) and circular 
in contour, but their orifices opening into the tracheids are lenticular and 
oblique. These pits are not nearly so tall as the height of the ray-cell, on 
the walls of which they are ranged in one or two tiers, and there are 2-3, 
usually two pits in each tier to the radial width of each tracheid, but only 
1-2 in the summer-wood—therefore, as a whole, 2-5 in the spring-wood 
(fig. 4). In the summer-wood the apertures are much elongated and oblique 
(fig. 3). The upper and lower walls, as well as the end walls, the latter 
especially near the margins, have rounded pits. 
The thin-walled parenchyma -cells are enlarged radially with straight 
transverse or oblique, or bow-like terminal walls, which are devoid of special 
thickenings or pits. 
The ray-tracheids have transverse or oblique ends, bordered pits on all 
their walls, and are devoid of denticulations or very feebly and scantily 
denticulate in the case of the marginal ray-tracheids. 
The wniseriute rays (fig. 5) are nearly entirely composed of thick-walled 
parenchyma and ray-tracheids, mainly the former. The latter are very 
largely marginal, but some internal ray-tracheids occur in smaller or larger 
numbers. Where the ray passes a resin-duct and comes in contact with the 
parenchyma surrounding this, thin-walled parenchyma partially replaces the 
thick-walled, but there are also a few thick-walled parenchyma-cells which 
are continuous with similar ones or with ray-tracheids, This change is quite 
local and the ray assumes the original structure after passing the duct. 
Occasionally biseriate rays devoid of resin-ducts may be seen. 
The fusiform rays (fig. 1) in tangential section are often slender, sometimes 
being one eell in thickness except in the central part where the resin-duct 
is. In tangential section the resin-duct is bounded by a wall of about four 
thin-walled flattened epithelial cells (which grow inwards to form thyloses). 
These are ensheathed by a single layer of. similar cells, whieh on the flanks 
are in contact with the wood-tracheids. These in turn are succeeded by a 
little collection of thin-walled wider parenchymatous ray-cells, usually in 
two tiers, but among them may be an isolated, thick-walled parenchyma-ray 
cell. The thinner portions of the ray, which are biseriate towards the centre 
