ORGANIZATION OF THE FOSSIL PLANTS OF THE GOAL-MEASURES. 885 
protoxylem canals.* This is a familiar process in recent plants, where parenchymatous 
tissues show active dilatation ; we may specially cite the case of the tuberous roots of 
Thladiantha, formerly described by one of us.t 
The structure of a principal medullary ray, as seen in a tangential section cut near 
the pith, is fairly constant in the various specimens examined. The middle rows of 
cells of the ray, varying in number in different cases from 2 to 8 or more, are 
short, often isodiametric, and sometimes tangentially dilated. The cells at the 
margins of the ray, adjoining the fascicular wood, are much elongated, but generally 
have square ends, like the rest (see Plate 79, fig. 15). If another tangential section, a 
little further to the exterior, be examined, we find that the principal rays are narrower ; 
the elongated marginal cells have pointed ends, or are replaced by tracheides. The 
middle short-celled part of the ray shows little change (see Plate 79, fig. 16). A third 
section, taken still further towards the outside of the wood, shows a more profound 
change of structure. The principal rays are here no longer continuous throughout the 
internode, but are partitioned up into a number of short, lenticular rays by trachee, 
or strands of tracheze, which cut obliquely through the original ray (see Plate 79, 
fig. 17). The same process is carried still further as we advance yet more towards the 
exterior, until, in some cases, the principal rays can scarcely be recognized any more, 
but are completely broken up into short rays, one or two ceils in thickness, which 
differ but little from the secondary rays of the fascicular wood.t{ 
The comparison of transverse sections confirms the above observations. Tracing 
the radial cell-series of the principal ray outwards, we find, not only that the marginal 
rows of parenchymatous elements are succeeded by rows of trachez, but also that new 
rows of tracheze make their appearance at various places in the interior of the ray. 
The latter sometimes appear to form the direct outward continuation of parenchy- 
matous series, while sometimes they are interpolated between them ; in the second 
case some of the parenchymatous radial series die out altogether.§ 
Favourable radial sections, which exactly follow the course of a principal ray, may 
also show how the more elongated parenchymatous cells are succeeded, towards the 
exterior, by trachee.|| 
There are many variations in detail; sometimes, for example, all the cells of the 
ray become more elongated towards the exterior of the wood. A more important 
* This is very conspicuous in C.N. 123*, where the inner part of each strand of wood is quite crushed. 
[This evidently took place during life, for the vest of the tissue, including the dilated ray-cells, is per- 
fectly preserved. | 
+ Scorr and Bresnur, “ Internal Phloém in Dicotyledons,” ‘ Annals of Botany,’ vol. 5, 1891. 
+ The above outline description is founded primarily on the series C.N. 65-68, from which the figures 
are taken, and has been confirmed by the study of several other series, as C.N. 20a and s, C.N. 83-87, 
C.N, 88-91, C.N. 130* and 131*, and C.N. 137* and 138*, as well as by that of single tangential sections, 
which are scarcely less instructive, when, as often happens, they are very slightly oblique, so as to pass 
gradually from an inner to a more external region of the wood, as in C.N. 1937, dc. 
§ See especially WinLtamson, “ Organization,” Part I., Plate 25, figs. 17 and 18. 
|| As in C.N. 132*#*. 
