414 DH. P. GROOM AND MH. W. RUSHTON ON THE STRUCTURE 
According to Gamble’s statistics the wood of Pinus excelsa is lighter in 
weight and less hard than that of Pinus Gerardiana : this is due to the thinner 
walls of both spring- and summer-tracheids, and perhaps to the smaller 
percentage of summer-wood in the former. 
The tracheids have pointed ends except where they end abruptly on 
medullary rays. In the spring-wood the numerous pits on the radial walls 
are nearly entirely uniseriate, but oceasionally paired, They are oval and 
transversely elongated or circular, with a transverse oval or nearly circular 
orifice. The torus is ragged at the edges and the membrane radially striated. 
Large simple pit-like structures oceur between some of the bordered pits 
(fig. 10). In the summer-wood the pits on the radial walls are uniseriate, less 
numerous, and circular in outline, with steeply oblique and even nearly 
vertical orifices (fig. 15). Sanio’s rims are narrow but quite distinct in the 
spring-wood. 
In the outer summer-wood on the tangential walls there are present 
numerous small scattered bordered pits. These are most abundant in the 
outermost layer of the tracheids, but may be seen in the outer three lavers. 
They are circular in outline, with nearly vertical-elongated fusiform apertures 
(fig. 14). 
Radial bars, * Sanio’s bars,” occur scantily: each is usually a cylindrical 
rod with a minute canal in its centre, 
Tracheids with the so-called * resin-plates?' occur against some of the 
medullary rays. 
Resin-Ducts. 
Each resin-duet has a flattened layer of thin-walled epithelial cells, which 
are broadly briek-shaped in longitudinal tangential view. In our specimen 
all the resin-duets were blocked by numerous large thyloses. Around the 
epithelium are parenchymatous cells whose form varies with the distance 
from the epithelium. The cells adjoining this are elongated brick-shaped 
with thin walls, and large rounded simple pits. Outside these the cells are 
longer, narrower, and still square-ended ; their walls are thicker and 
lignified, and the simple pits are smaller ; against the tracheids the pits 
are half-bordered. Finally there are parenehyma-tracheids whose terminal 
walls are transverse or somewhat oblique ; these have small bordered pits on 
all their walls. ‘The parenchyma extends farther in a tangential direction 
towards a medullary ray than towards the reverse side; moreover, this 
production of wood-parenchyma in contact with the medullary ray may be 
continued on the side of the latter that is distant from the duct. 
Medullary Rays. 
The uniseriate rays are distinctly shallower than the fusiform rays, as 
they vary from 1-12 cells in height, whereas the latter rays are often 
equivalent to 18 or 19 cells in height. 
