574 Bower.—On Medullotion in the Pteridophyta . 
venient indicator of tissue locality, and it has been seen that not only the 
tissues outside it, but also those interior to it may take their varying share 
in the process of medullation. 
DESCRIPTION OF THE FIGURES IN PLATE XLVII. 
Illustrating Professor Bower’s paper on Medullation in the Pteridophyta. 
Selaginella spinulosa , A. Br. 
Fig. i. Transverse section of the stele of Selaginella spinulosa , taken from the base of the 
strobilus, showing the central xylem-core composed of tracheides, with projecting protoxylem- 
groups. Occasional thin-walled elements are present in the xylem. x 72. 
Fig. 2. Transverse section from rather below the middle of the strobilus, showing the general 
arrangement as before, but there are now considerable but irregular tracts of thin-walled elements, 
especially towards the centre of the stele, which are connected irregularly by thin-walled rays with 
the outer conjunctive tissue. There is, however, no evidence of intrusion shown in the arrangement 
of the cells, x 72. 
Fig. 3. Transverse section from about two-thirds up the strobilus, showing the general arrange¬ 
ment as before, but the whole of the central region is occupied by thin-walled elements, constituting 
a massive pith. The xylem-core is now broken up into a number of separate strands, with broad 
parenchymatous rays intervening between them, x 72. 
Fig. 4. A stage corresponding approximately to that of Fig. 2, on a larger scale, and showing a 
single tracheide isolated in the centre of the parenchymatous tract, x 144. 
Fig. 5. A stage corresponding approximately to that of Fig. 3, on a larger scale, showing the 
relation of the isolated xylem-strands to the central pith. x 144. 
Fig. 6. Part of a stele from a level about half-way up the strobilus, showing isolated tracheides 
suggestive of the degradation of the xylem-core. x 325. 
