Knowledge of Lyginodendron. 69 
The dark concentric lines seen in a transverse section of the 
wood, and especially well marked on the right-hand side of 
the photograph (Fig. 1), are for the most part due to the 
occurrence of lines of narrower tracheids, locally developed in 
response to some influence which was much too irregular to 
be the result of seasonal changes. In Fig. 2 the wood and 
pith are shown in radial section ; the mottled appearance of 
the former is due to the numerous and large medullary rays 
which form a characteristic feature of the stem. In Figs. 3 
and 4 the character of the wood is more clearly seen ; the 
lower end of Fig. 3 is in the immediate neighbourhood of 
the inner limit of the wood, and here the tracheids are fewer 
in number and farther apart than in the more external portion 
of the wood. The spaces between the narrow and curved 
rows of tracheids correspond to those seen less distinctly in 
Figs. 1 and 6 : in this region of the stem the medullary-ray- 
parenchyma has usually disappeared. 
The xylem is entirely composed of tracheids with reticu- 
lately-pitted radial walls (Fig. 5) ; the nature of the pitting 
on the tracheids which form the innermost limit of the wood 
cannot be clearly seen owing to imperfect preservation. The 
xylem-elements are arranged in radial rows varying in breadth 
from one row of tracheids to bands composed of eight rows. 
In tracing bands of tracheids towards the periphery of the 
stem the number of elements composing any one band in 
a tangential direction is found to vary considerably, as the 
result of the intercalation of new rows or of a fusion between 
adjacent bands. The medullary rays consist of radially 
elongated parenchymatous cells of one to ten rows in breadth, 
the average breadth of a ray being nearly equal to that of the 
tracheidal bands. The broad medullary rays give to the wood 
the characteristic appearance noticed in the description of 
Fig. 2 . Towards the inner margin of this wood the medullary- 
ray-cells are usually absent, but in places where they have 
been preserved they are found to be tangentially elongated 
follow a gradually ascending and steep course, very similar to that of the leaf-trace 
bundles of Lyginodendron Oldhamium. 
