266 DR R. KIDSTON AND PROFESSOR D. T. GWYNNE-VAUGHAN ON 
activity of a cambium of some kind, and the general appearance and structure of the 
tissue indicates it is best regarded provisionally as a deep-seated and probably thick- 
walled periderm. A certain amount of phloem and other soft tissues may have existed 
between this zone and the xylem, but if so they have entirely disappeared. In any 
case such tissues could not have been very extensive, for, after making every allowance — 
for crushing and imperfect preservation, the dark-celled zone fits very closely into the 
xylem,—so much so indeed at some points as almost to suggest that it might have arisen — 
from the same cambium. 
Towards the outside of this dark-celled zone its elements become larger and 
irregularly polygonal in outline, the radial arrangement being in consequence lost 
(fig. 16, 0.z.). In addition, large groups of similar cells form rounded bosses, projecting 
at irregular intervals beyond the general level of the outer surface of the zone (fig. 16, 
b.). The same type of cell also forms a number of scattered spherical masses in the 
cortex of the stem (fig. 17). The central cells of these masses, and also those of the 
bosses, are smaller than the others, and form a sort of nucleus from which the larger cells" 
radiate more or less distinctly, appearing as though, during their development, they had 
been pulled out by the growth of the surrounding tissue. Their cell walls are very im- 
perfectly preserved, but it is very probable that they were of a sclerotic nature. It is 
clear that the masses they form were during life quite hard and resistent, for whenever 
they have been pressed against a mass of xylem, it is the latter, and not the former, that 
has given way and become crushed. The “sclerotic nests,’ as they may be called, are — 
scattered without order throughout the inner cortex of the stem, and do not seem to 
bear any relation to the leaf-traces. In structure and appearance they are quite similar 
to the sclerotic nests described by Dr Scorr in the pith of Calamopitys Beinertuanum.* 
The outer cortex of the stem is of the “ Sparganum” type (fig. 18). That is to say, 
it contains a number of rather short radiating bands of fibrous sclerenchyma, running — 
vertically almost without anastomosis (fig. 19, from the petiole). The tissue between 
these sclerotic bands is a rather firm-walled parenchyma. Some of its cells have dark- 
coloured contents, and may perhaps be regarded as resin sacs. 
The Departure of the Leaf-trace. 
The leaf-traces depart from the ends of the lobes of the primary xylem in a perfectly 
protostelic manner. Our series enabled us to follow one such departure from start to 
finish. First the extremity of one of the xylem lobes is seen to become more and more 
prominent (figs. 6-7, /.t.”). Then the protrusion is gradually nipped off as a fairly large 
roundish leaf-trace (figs. 8-9, /.t.2). As the trace passes out it carries with it some of 
the secondary xylem on its abaxial side, and at the same time it also obtains a certain 
amount of secondary xylem of its own on the adaxial side (figs. 9-10). The depar-— 
* Scorr, “On the Primary Structure of certain Paleozoic Stems with the Dadoxylon type of Wood,” Trans. Roy. 
Soc. Edin., vol. xl. p. 342, pl. i. figs. 3-4, 1902. 
