~N 
OF THE WOOD OF INDIAN SPECIES OF PINUS. 48 
Medullary Rays. 
The uniseriate rays attain a maximum height of 17 cells: the fusiform 
rays are relatively shallow, in fact not so tall as the highest uniseriate rays. 
The ray-tissue includes, apart from the resin-ducts, three extreme types of 
cells : thin-walled and thick-walled parenchyma, and ray-tracheids. 
The parenchyma is mainly of the thick-walled type. The terminal walls 
are more or less transverse and include a few rounded simple pits. On the 
lateral walls, to the radial width of one spring-tracheid there are 1-5 large 
pits, which are in one tier if 2-5 in number, and are in two tiers if 4-5, 
When, as is usual, there is only one tier of pits on the radial wall, the height 
of the pits is not much less than that of the cell. The pits vary in contour 
from oblong or radially elongated to circular or oval vertically elongated 
(fig. 50). In the summer-tracheids on the lateral walls there is only one pit 
to the radial width of one tracheid, and this pit has an oblique oval or a 
circular outline with a long, narrow, nearly vertical, fusiform aperture, 
towards the wood-tracheid (fig. 45). The upper and lower walls have 
numerous simple pits of rounded outline. In some of the rays markedly 
narrower and broader parenclyma-cells of this type are to be distinguished. 
The usual length of a cell is equal to the radial width of 4—5 spring-tracheids 
or 4-6 summer-tracheids. The lateral walls do not bulge into the contiguous 
g. 39). 
The thin-walled parenchyma-cells are radially elongated with walls that are 
tracheids. In tangential section the cell is oval, erect or circular (fi 
not pitted, 
The ray-tracheids have denticulate walls, and their ends are straight and 
transverse or oblique, or are convex. The walls often are locally thickened 
to such an extent that in tangential sections the ray-tracheids present the 
appearance of fibro-tracheids (see fig. 39). Marginal and internal ray- 
tracheids occur, and the former often assume peculiar forms, especially in the 
summer-wood, as the terminal part emits a lateral process which either grows 
over the next ray-tracheid (fig. 41) or forms a more or less erect cecum, In 
the latter case the ray-tracheid in shape and direction is transitional towards 
a wood-tracheid that applies its bent end to a medullary ray. When passing 
а resin-duet the ray-tracheid may be replaced by a thick-walled parenchyma- 
cell, but ray-tracheids are also found crossing a resin-duct : possibly, in this 
and other species of Pinus, the change from ray-tracheid to parenchyma does 
not take place if the ray is passing a point at which parenchyma-tracheids 
are in contact with the ray, but does take place if the ray comes into 
direct contact with parenchyma around the duct. 
The wniseriate rays vary considerably in composition. АП four types of 
cells oceur in them, and the ray-tracheids are internal as well as marginal. 
One ray for instance showed the following composition (the Roman numbers 
