OF THE WOOD OF INDIAN SPECIES OF PINUS. 483 
The ducts in place of being solitary are usually clustered in groups of two, 
three or even more. Frequently a medullary ray divides the cluster, which 
often includes two kinds of duets—normal wide ones and much narrower ones. 
The diameter of a large resin-duct measured was nearly 80 and the radial 
diameter of the epithelium-cells nearly 184. The duct itself is walled by a 
single layer of flattened peripherally extended thin-walled cells which tend 
to be somewhat elongated in a longitudinal direction. Around these is a 
collection of thin-walled parenchyma which is more extensive than usual. 
These cells are more elongated than those walling the duct, and towards the 
surrounding tracheids are increasingly narrow and long, their walls not 
being lignified. Where a resin-duct is close to a uniseriate medullary гау, 
this parenchyma may also form a radial series on the distant side of the ray. 
Outside the thin-walled parenchyma there occur two other types of cells : 
thick-walled parenchyma and parenchyma-tracheids. Sometimes the latter 
are separated from the thin-walled parenchyma by the former (fig. 31). The 
thick-walled parenchyma-cells are elongated, and on their longitudinal walls 
occur numerous more or less broad or narrow fusiform simple pits, which are 
elongated transversely or obliquely. The parenchyma-tracheids are shorter 
than ordinary tracheids and their bordered pits are considerably smaller than 
those of the surrounding normal tracheids. Where a tracheid abuts on 
parenchyma the pit is half-bordered. 
Medullary Rays. 
The uniseriate rays may reach a height of twenty cells, and the fusiform 
scarcely exceed this or are shallower. Apart from the resinous epithelium, 
the rays are mainly composed of ray-tracheids and thin-walled parenchyma, 
though parenchyma with somewhat thicker walls is not entirely lacking. 
The ray-tracheids are dentieulate, bear bordered pits on all their walls ; 
the terminal walls are transverse or oblique. The length of the ray-tracheid 
equals the radial width of 3-4 wood-tracheids in the spring-wood, and about 
6 in the sammer-wood. 
The thin-walled parenchyma-cells mainly have transverse terminal walis, 
and in length are equal to the radial width of 5—6 tracheids in the spring-wood, 
and 3—4 in the summer-wood. On the lateral walls the pits are large and 
only little less high than the cell itself (fig. 27). There is usually one 
pit to the radial width of a tracheid, both in the spring-wood and the 
summer-wood, and in the latter the shape varies from oblong-oval with 
the long axis usually horizontal to oblique broadly fusiform. In the spring- 
wood the large pit may be divided into twọ or rarely into three or four small 
ones, whose contours are fusiform oval or nearly circular. Passing to the 
summer-wood, the aperture of the single large pit on the side towards the 
