50 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
These peculiarities, which recur in every one of the Burntisland specimens, result in an 
absence of that regular parallelism of the successive concentric layers which renders the 
young states of the Oldham plant so remarkable for their symmetric beauty. The bark 
now distinctly exhibits its separation into an inner (g), a middle (h), and an outer layer (Jc). 
The tissues of the two former are almost entirely destroyed, but the latter obviously con- 
sists of the thick-walled prosenchyma characterizing the Lancashire examples. A series 
of intermediate conditions conducts us to the magnificent stem of which a section is repre- 
sented in Plate IV. fig. 21. This stem, inclusive of the bark, has a maximum diameter of 
•34, that of the ligneous zone is ’25, that of the primary, or inner, circular zone is -091, 
and that of the primary triangular axis is *063. All the large central vessels of the tri- 
angle have a maximum diameter of -0053, or nearly double the size of those constituting 
the corresponding portion of the young twig, Plate III. fig. 18. The most conspicuous 
features to be noticed in this noble section are the extreme distinctness of the change 
in the growth of the exogenous layers, to which I have already directed attention when 
describing the Oldham example, Plate II. fig. 11, and a distinct lacuna near the ex- 
tremity of each arm of the central triangle, indicative of a longitudinal canal at each of 
those points. We here see that the primary exogenous additions were made in the 
way already illustrated by Plate III. fig. 20. But when the ligneous axis of this stem 
attained an almost perfectly cylindrical form and a diameter of ‘091, it experienced, 
through some unknown agency, an interruption to the regular continuity of its 
exogenous growths. Whatever the cause of this check to vegetative action was, it 
passed away, and circumferential growth was resumed, but under some altered con- 
ditions. The new laminae continued to be developed along radiating curves, which 
followed the same directions as before ; those which were continuous with the three 
sets of small vessels that diverged from the corresponding angles of the central 
triangle were little, if at all, altered ; but the case is otherwise with the three 
intervening portions. Here the large vessels of the primary exogenous layers terminated 
suddenly, giving place to very small ones ; so that the innermost portions of the newer 
growths consisted of concentric vascular zones composed of vessels of nearly equal size 
throughout the greater part of each ring. As further successive additions were made, 
these new wedges became slowly broader, until, at their peripheral portions, the 
vessels of such as radiated from the concavities of the central triangle opposite to d d d 
became almost as conspicuously larger than those proceeding from the angles of the 
triangle opposite to d ' d' d\ as was the case in the more central primary portions of the 
woody axis. 
The bark in this stem displays very clearly its separation into an indistinctly double 
inner layer (g and h) and an outer one (Jc). We also see very distinctly, especially in the 
former portion, the individual cells of which it is composed; but these have been flattened 
and more or less detached from each other by pressure or mineralization, so that their 
primary arrangement is indistinctly indicated. As in the younger branches, the inner 
and middle barks have suffered more from these causes than the outer one. 
