716 PROFESSOR W. C0. WILLIAMSON AND DR. D. H. SCOTT ON THE 
secondary rays appear de novo in the later-formed layers, as secondary growth proceeds 
(see fig. 1, &c.). The proportion of rays to wood and bast varies much in different 
specimens, arid in different parts of the same specimen. On the whole, the interfasci- 
cular wood is richer in ray-tissue than the fascicular, but this is not a constant rule. 
All the medullary rays are continuous through the cambium into the phloém, so that 
we can speak of xylem-rays and phloém-rays just as in the case of recent trees (see 
Plate 22, fig. 7). 
The structure of the secondary wood is excessively simple; it consists of tracheides 
and ray-parenchyma only. The tapering ends of the tracheides can often be clearly 
seen, and sometimes it is evident that the pits are closed throughout, so that we have 
direct evidence that the xylem elements were really tracheides, and not vessels (see 
Plate 22, fig. 4B). The tracheides are of great length, but we have not attempted 
measurements, as we could not make sure of following the same element throughout 
its whole length, 
The tracheides have very numerous pits, which usually, if not always, are limited 
to their radial walls. The pits are crowded together, and as many as seven longitu- 
dinal rows may exist on the same wall. They show signs of an arrangement in 
inclined series, as if they formed part of a spiral system. The pits are distinctly 
bordered, as can be seen both in tangential and radial sections. The opening of. the 
border is a wide inclined slit ; the pits between tracheides and ray cells are, as usual, 
unbordered on the side towards the latter (see figs. 44 and 48). 
The rays are of very variable height and width; their cells are decidedly thin- 
walled, and consequently are only well-preserved in good specimens. They are radially 
elongated, and thus show in radial sections the muriform appearance characteristic of 
most medullary rays in recent plants.* 
The cells with dark carbonaceous contents, which we interpret as secretory sacs, 
and which are so general in all parenchymatous tissues of Lyginodendron, frequently 
occur in the medullary rays. 
The secondary phloém has a very characteristic structure; it is divided up into 
small groups corresponding to the tracheal groups in the wood, and separated from one 
another by the parenchymatous phloém-rays (see Plate 22, fig. 7 and 74). In each radial 
series of phloém elements, there is a regular alternation of larger and smaller cells 
(see fig. 7). The tangential section appears to show that the smaller elements of 
the phloém were rather elongated, with occasional transverse walls, while the larger 
elements had oblique terminations, Great caution, however, is necessary in inter- 
preting the structure, on account of the presence of articulated fungal hyphe in the 
partially disorganized phloém (see fig. 7A). 
The preservation does not allow us to say for certain which are the sieve-tubes. 
We think it most likely that the larger elements were of this nature, while the 
* See Wittiamson, ‘ Organization,” Part IV., Plate 23, fig. 9; also our fig. 4. 
