[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 395. 
plate is in the form of a narrow band running right around the bundle 
and curving in and out in a zigzag manner. There is thus an alterna- 
tion of concave and convex portions of the xylem plate on each side of 
the bundle. On the inner side of the xylem curves whose concave faces 
lie towards the inner side of the bundle are situated the protoxylem 
groups, the thyloses being in this position in the sections examined 
(Photo 14). On the inner side of the curves whose concave faces lie 
towards the outer side of the bundle, and thus alternate with the proto- 
xylem groups, are groups of tannin- or resin-bearing cells. The num- 
ber of groups of thyloses in a bundle varies with the size of the bundle. 
In a section of the stipe which was 1 em. across, 35 distinct groups of 
thyloses were counted, in one which measured 0.5 cm. across, 26 groups 
were present. 
Each group usually consisted of from 3 to 6 large thyloses which 
were rather thin-walled and poor in contents. Broken protoxylem 
elements were also frequently observed in these. In many cases the 
groups of thyloses presented the appearance of vessels or cysts with 
epithelium, so that, as far as could be judged from the material available, 
the structure here exhibited parallel development to that in Pteris 
aquilina. 
In Cyathea medullaris also, only the rachis was available for exam- 
ination. In transverse sections of this organ three bundles are seen, one 
large one, and two smaller ones above it. The bundles have the general 
appearance of the strap-shaped bundles found in the oldest stipes of 
Pteris aquilina which are formed by the fusion of several smaller ones. 
In each bundle there are several large groups of thyloses adjacent to the 
protoxylem groups (Photo 15). The individual thylose cells are very 
large, and in the sections examined were thin-walled and frequently 
showed prominent nuclei. Rings and spirals of the disorganised proto- 
xylem elements were also evident in many of the groups. Thyloses 
which are modified to simulate the appearance of vessels with epithelium 
were found to occur here frequently, and in these it was clearly seen 
that the original wood parenchyma sometimes formed part of the 
epithelium. 
Todea barbara and Osmunda regalis were the forms examined in 
the Osmundacee. 
In sections obtained from a young stipe of Todea barbara before 
the pinnæ had become unfolded, it was seen that all the tissues were in 
a very immature state. The cells which stand out most distinctly in the 
vascular bundle at this stage, are groups of four or more brown or red- 
dish coloured cells which lie at frequent intervals mainly on the inner 
