1057 
Affinities of Sutcliffia. 
protostele 1 surrounded by large, irregular masses of vascular tissue, detached 
from or in connexion with the main stele. The origin and subsequent 
behaviour of these vascular masses (‘ meristeles ’) is very similar ; in both 
cases they are derived by the separation of portions from the protostele, 
and in both these detached portions undergo further irregular division. In 
vS. insignis the meristeles ‘ form the points of departure for the actual leaf- 
traces ’, 2 while in our specimen evidence is afforded that they are entirely 
used up in the production of foliar bundles. 
Fusion between ‘ meristeles, probably of quite distinct origin } , occurs 
only in the type specimen, but re-fusion of ‘ meristeles ’ with the meri- 
steles of similar origin, and with the protostele, has been described for our 
fossil (p. 1039). In both cases the ‘ meristele 5 repeats the structure of the 
protostele on a small scale. 
The leaf-trace strands form a further feature of comparison between the 
two specimens, for in both stems the foliar traces are concentric and the 
structure is exarch, and, moreover, two types of leaf-trace bundles are 
present in both, namely, the unilateral and the radially symmetric. 
Finally, the agreement in histological details between our stem and 
that of 5 . insignis is very striking. The structure and composition of the 
primary wood is very similar in the two cases, and the beginning of 
secondary growth in 5 . insignis agrees closely with the first formed secondary 
tracheides in the new specimen. The primary phloem surrounding the 
leaf-trace bundles shows the same peculiarities in both stems. The strands 
of fibrous elements with the closely associated secretory elements, and the 
great abundance of secretory cells throughout the tissues, are characteristic 
features common to both. 
Thus, a consideration of the general structure of the stem, the origin 
and behaviour of the ‘ meristeles ’, the leaf-trace bundles, and the close 
agreement in histological details in the two stems leaves little room for 
doubt that the relationship between them is exceedingly close. The question 
next arises whether the differences which occur are sufficient to warrant 
specific distinction, or whether they can be explained on any other grounds. 
The outstanding feature which characterizes our stem is the great 
development of the secondary tissues around the stele and ‘ meristeles 
coupled with the network of extrafascicular zones and accessory vascular 
strands ; the absence of any leaf-base forms a further feature of difference. 
The latter is undoubtedly to be correlated with the formation of periderm ; 
in Stitcliffia insignis cork was absent, though, as Scott has pointed out, 3 
arcs of meristem were in process of formation ; so the absence of leaf-bases 
may therefore well be attributed to the greater age of the stem. It appears 
Using the term in the sense in which it is employed in Tansley’s ‘ Lectures on the Evolution 
of the Filicinean Vascular System'. New Phyt., 1908, p. 138. 
2 Scott: loc. cit., p. 54. 3 See footnote, p. 1052. 
