ORGANIZATION UF THE FOSSIL PLANTS OF THE COAL-MEASURES. ras: 
photograph 2, shows the structure at a very early stage. Similar specimens have been 
observed with little or no secondary wood, but these are always in a somewhat 
fragmentary condition. 
That the zone of wood and bast, superadded to the primary bundle system, was 
really of a secondary character, is conclusively proved by well-preserved specimens in 
which the actual cambium is often quite evident (see Plate 18, photograph 1, Plates 21 
and 22, figs. 1,5, and 7). It is also manifest that the cambium was a normal one, 
forming wood on its inner and bast on its outer surface. We leave out of consideration 
for the moment certain individual anomalies, which we shall describe below (p. 722). 
The cambial cells, which form the xylem and phloém elements, were of the 
usual tabular form, as seen in transverse section (see fig. 7). In the development of 
the medullary rays the tangential divisions were no doubt less frequent, for the 
cambial cells in this region have a greater radial diameter. The general character 
of the secondary growth resembles that of the stem in the Cycads. 
In the very young stem, shown in photograph 2, it will be noticed that the thin 
zone of secondary wood, whether fascicular or interfascicular, is of about the same 
thickness all round the stem. It evidently formed, from the first, a continuous ring, 
only interrupted at the point of exit of a leaf-trace bundle. Hence we must infer 
that the interfascicular cambium began its activity almost simultaneously with that 
in the bundles themselves (see also photograph 1 and fig. 1). 
It is clear that the cambium followed the course of the outgoing bundles for some 
little distance, for these bundles have their own ares of secondary wood and bast up 
to a certain point in their outward course. The secondary arc dies out as we trace the 
bundle upwards (see Plates 21-23, fig. 1, .t.! and 1.t.°, figs. 5 and 9). Below this point, 
however, the cambium has already closed in behind the outgoing bundle, so that 
for a certain distance there is a double layer of cambium. There is the special arc 
of cambium belonging to the leaf-trace bundle itself (fig. 5, cb.””), and, besides this, 
there is the general cambium, which is continuous behind it. The parenchyma, 
occupying the gap behind the leaf-trace bundle, where the cambium had not yet 
closed in (see figs. 1 and 9), followed the secondary growth by dilatation, accom- 
panied, no doubt, by irvegular cell-divisions. Some curious anomalies which appear 
in this region are described below (p. 723). 
The whole secondary zone is made up of radiating lamine of xylem and phloém, 
with medullary rays between. We see no reason for departing here from the usual 
terminology, which would be applied without hesitation to the stems of Cycadez, the 
plants which most nearly resemble Lyginodendron, so far as the secondary tissues 
are concerned. 
The primary medullary rays usually became divided up at once by the formation 
of interfascicular laminze of wood and bast (see photographs 1 and 2, fig. 1, &c.). 
Conversely the wood and bast, which are formed opposite the primary bundles, are 
from their first origin subdivided by secondary rays (see figs. 2and 3). Additional 
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