884 PROFESSOR W. CG. WILLIAMSON AND DR. D. H. SCOTT ON THE 
tissue, or at least (7.c., in the case of uniseriate rays) form continuous radial series. 
The fascicular wood consists of traches and medullary rays only. 
The greut. differences in shape between the cells of the medullary rays suggest the 
possibility of a physiological differentiation, such as has been found by STRASBURGER 
in certain Dicotyledons, in which the upright and horizontal elements of the rays 
differ considerably in structure, and presumably in function also.* But we have no 
evidence of this in the fossil. Neither have we any reason to suppose that tracheides 
were present in the secondary rays, as is the case in the Abietine, although the 
general absence of tangential pits in the secondary wood might have led us to expect 
such an arrangement.t Only the nature of the pitting can guide us in such a question, 
and the evidence available is not conclusive, for such details can only be adequately 
studied in the best preserved specimens. 
Pits on the walls separating the cells of the medullary rays from one another are 
seldom shown at all clearly. These cells usually have rather thin walls, and such pits 
as they may have had could have been of no great depth. 
It now remains for us to consider the structure of the principal medullary rays, and 
more especially to investigate the process by which, in most of our specimens, they 
become bridged over by interfascicular wood. 
The principal rays in the type (B 2) which we are now considering are of maximum 
breadth next the pith, and taper off rapidly (as seen in transverse section) towards the 
exterior (see Plate 72, photographs 2 and 3; Plate 77, fig. 3). The width of the inner 
end of the principal ray is much increased, in the older specimens, by the tangential 
dilatation of the more internal cells of the ray.{ We find this phenomenon in many 
of the older stems,§ but its occurrence is inconstant ; the dilatation may take place in 
some rays and not in others, within the same transverse section. The tangential 
width of a dilated ray-cell may amount to ‘25 millim. or more, which is from two to 
three times that of the neighbouring unaltered cells. It is the middle cells of the ray 
which become dilated ; those adjoining the wood on either side remain unaffected. 
It is evident that so considerable an extension of the width of the medullary rays 
allowed of a perceptible enlargement of the pith-area during the earlier stages of 
secondary growth. It is probable that the circumference of the pith may have 
increased in this manner to 14 times its original extent, or even more. 
It can be proved that the tangential extension of the inner ray-cells was an active 
process, and not simply due to the tensions set up by the growth of other tissues. 
This is shown by the fact that the dilatation of the rays sometimes led to the crushing 
of the wedges of fascicular wood between them, and thus to the obliteration of the 
* Srrasporcer, ‘ Histologische Beitrige,’ vol. 3, p. 209. 
+ See Srrassurawr, loc. crt., p. 9. 
t See the figure in Wittiamson, ‘‘ Organization,” Part I., Plate 27, fig. 26, which, however, only shows 
a small degree of dilatation compared with many larger stems, 
§ As in O.N. 133,* &c. 
