200 
M. A. Chrysler. 
have been figured by Penhallow 1 for the stem wood of P. refiexa. 
It would be interesting to ascertain whether the wood of other 
parts of the tree in this species shows well-marked tyloses. 
Although these structures are best shown in a tangential 
section, they may be seen in transverse section also, as is represented 
in figures 3 and 4. In either case they are rendered much more 
easily visible by the application of a hematoxylin stain (especially 
Ehrlich’s), and may in fact easily be overlooked in an unstained 
preparation, on account of the delicacy of their walls and because 
they fit so tightly against the walls of the tracheid. Fig. 3 shows 
early stages in the formation of tyloses as are commonly exhibited 
in sections through the heart wood of roots of P. strobus. The 
figure shows both central and marginal cells of two medullary rays, 
and the protrusions from the pits of the former sort. In one 
corner is visible part of a resin canal with its usual complement of 
tyloses. Apparently in this species the tyloses do not attain the 
development shown in several other species, for in material collected 
in localities as far apart as Cambridge, Mass, and near Georgian 
Bay, Ontario, no well-developed cases were found. Near the pith 
of roots of P. strobus the outgrowths are frequently exceedingly 
numerous, but are not confined to this region, for they extend 
throughout the heart wood, becoming scarce as the sap wood is 
approached. They may occur in any part of an annual ring, and 
are not more numerous in spring wood than in summer wood. 
Further stages in the growth of a tylosis are shown in Figs. 
5 and 6. The tylosis increases in length until in some instances it 
extends to the end of a tracheid, as may be seen in Fig. 6, which is 
from a tangential section through the first year’s growth of a cone 
of P. ayacahuite. The tylosis in question has also extended into an 
adjoining tracheid by way of one of the pits. The same tylosis 
illustrates, though not very clearly, another stage of growth, namely 
the appearance of a cross wall. Evident instances of cross walls 
have been observed in other species, for example, P. rigida, but this 
stage does not seem to occur by any means as frequently as in the 
tyloses found in the vessels of dicotyledonous woods. Fig. 5 
illustrates in a diagrammatic way the appearance and origin of the 
abundant tyloses in the root of P. palustris. What may be regarded 
as the final stage in growth is the more or less extensive filling of a 
tylosis with a resinous material. This is illustrated by the 
photograph of a transverse section through the cone axis of 
1 Penhallow, D. P. A manual of the North American gymno 
sperms. Boston, 1907. 
