750 PROFESSOR W. C0. WILLIAMSON AND DR. D. H. SCOTT ON THE 
in Cycadesx. It is quite evident that the protoxylem is not peripheral, and that the 
centrifugal elements exterior to it still belong to the primary wood.* 
These definite xylem-bundles, each with its own protoxylem, do not always occupy 
the whole periphery of the primary wood. They are often separated from one 
another by portions of the same irregular tracheal groups which extend throughout 
the whole interior of the cylinder. For convenience of distinction, we propose to 
apply Van TrecuEm’s term “ metaxylem” to these latter groups, reserving the term 
“primary xylem-strands” for those peripheral bundles which are continuous with the 
leaf-traces. Usually, however, no sharp limit can be drawn between the two, for, as 
a rule, the xylem-strand passes over gradually on its inner side into the general 
metaxylem of the cylinder (see Plate 26, fig. 24, #. and m.x.). 
It may be mentioned, that in the specimens of H. Girievii from Lancashire, the 
protoxylem-elements lie somewhat nearer the external surface of the xylem than in 
those from Burntisland. 
We have no information to give as to the structure of the primary phloém. 
Remains of soft tissue in a more or less disorganized condition can always be found 
between the wood and the pericycle, but the structure of this zone is never preserved 
in stems without secondary thickening. 
The pericycle is sharply marked off from the disorganized phloém-zone; the latter 
has a characteristic brown tint, while the pericyclic cells are clear (see Plate 27, 
fig. 26, p.c.). The short-celled pericyclic tissure is often well preserved, though its 
cell-walls are thin. In the Dulesgate specimens the pericycle is traversed by 
secretory sacs, which are less evident in the original Burntisland form. 
It remains to consider the structure of the leaf-trace bundles on their outward 
course. 
Fig. 26 shows the transverse section of one of these bundles, which is just 
separating from the stele, but is still enclosed within the pericycle. The position of 
the protoxylem is not certain, but we believe, from the comparison of many sections, 
that there are two such groups at the points indicated, px. Some of the longitudinal 
sections pass through out-going leaf-trace bundles, and show clearly the position of 
the laxly spiral elements in the interior of the xylem strand.t 
The collateral structure of the bundle figured in fig. 26 is fairly evident ; the thin- 
walled phloém is tolerably well preserved and undoubtedly seems to be limited to 
the outer side of the strand. Unfortunately the preservation in H. Grievit is seldom 
* The best slides for showing the protoxylem and primary xylem-strands are :— 
Transverse, O.N. 1291, 1293, 1294 (Burntisland). 
1915 C. (Dulesgate). 
Longitudinal, C.N. 1266, 1268 A, 1276, 1286, 1288 (Burntisland), 
1915 G. (Dulesgate). 
+ The following preparations show leaf-trace bundles in longitudinal section: O.N. 1265, 1282, 1284, 
and 1915 G, 
