7o 
Seward. — A Contribution to our 
between the widely separate bands of xylem. In Figs. 7 and 8 
the wood is shown in tangential and radial section ; in the 
former the polygonal cells of the medullary rays are seen to 
occupy long and narrow meshes, usually with pointed ends, 
in a framework of tracheids. No trace of bordered pits has 
been noticed in the tangential walls of the wood-elements. 
In radial section (Fig. 8) the reticulate pitting is found to be 
fairly well preserved, but the borders of the pits are for the 
most part imperfect (Fig. 5); the medullary-ray-cells present 
the characteristic appearance of radially-elongated parenchy- 
matous cells, with here and there dark patches suggesting the 
contents of secretory cells. Seen under a higher power, 
the radial walls of many of the tracheids are found to be 
traversed by sets of parallel and obliquely running lines. It 
is difficult in many cases to determine how far such parallel 
markings on the xylem-elements of fossil plants are the result 
of cleavage in the infiltrated mineral substance, or to what 
extent they are the expression of some histological feature. 
In the tracheids of fossil coniferous wood we frequently find 
delicate and regular lines agreeing very closely in appearance 
with the striation characters of recent tracheids, and probably 
due to the emphasizing by ferment-action and decay of the 
original striations in the substance of the tracheid-walls. In 
the present instance it is probable that the lines referred to 
are in some measure due to the original striation of the 
tracheids. 
The structures hitherto described in detail constitute the 
secondary centrifugally-developed wood. If the rows of 
tracheids in the secondary wood be traced to their inner 
termination, they are found to taper off to an acute apex, 
or to merge into a small group of tracheids in which the 
radial arrangement is either entirely absent or indistinctly 
marked. As already pointed out, the medullary-ray-cells 
have usually been destroyed in this region, and the spaces 
separating the inner terminations of the tracheid bands are 
often bounded by a fairly regular line, concave towards the 
pith ; this marks the limit of decay, and recalls to some extent 
