688 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
The elements which adjoin the actual protoxylem on the inner side are often 
reticulated. Further towards the interior the primary trachez become considerably 
larger, and have pitted walls, the pits being often transversely elongated, so as to 
give a scalariform character to the thickening (fig. 3). Some of the pits show a 
slight border. The primary tracheze have very oblique terminal walls, which are 
pitted like the lateral walls; there is no reason to doubt that these elements were 
closed tracheides, and not open vessels. 
The primary xylem-groups, then, were evidently developed centripetally, and so 
far agree with those of recent roots. 
The primary strands of phloém can of course ouly be recognized in the most 
favourable preparations. A good example is represented in fig. 1, which is drawn 
from a section previously figured as a whole.* ‘The specimen is the original one 
of “ Myriophylloides Williamsoni,” described by Hick and CasH in 1881, and another 
transverse section of it is figured in the memoir by those authors.t Neither of the 
former figures, however, show either the phloém or the primary xylem. The details 
are remarkably clear in this specimen. The root has nine groups of primary wood, 
and shows the commencement of secondary growth; the secondary wood is as yet 
only three or four cells in thickness. Immediately outside the wood is a zone of 
very delicate tissue ; opposite the protoxylem-groups this zone is excessively narrow 
—only two cells in thickness—and evidently represents the dividing cambium only. 
Between the groups of primary wood, however, the delicate tissue attains a thickness 
of about five cells; evidently phloém is present at those places, to the outside of the 
cambial layer (fig. 1, ph, ph, on either side of the protoxylem, px). The alternation 
of the primary strands of xyiem and phloém is quite regular. Here, then, is another 
point, in which these organs conform to the typical structure of recent roots.{ 
Secondary tissues, as is already well known, were formed in great abundance, and 
the various preparations show them at all stages of development—the wood, in the 
larger specimens, sometimes attaining a radial thickness of at least 60 elements. 
* Witiiamson, “ Organization,” Part XII., Plate 28, fig. 2. 
+ Hick and Casu, /oc ctt., ‘Proc. Yorkshire Geol. and Polytechnic Soc.,’ 1881. 
{ Hick and Casu describe the whole of this thin-walled zone as cambium, loc. cit., p. 402. These 
authors have expressed doubts as to the identification of their Myriophylloides with Astromyelon (loc. cit., 
‘Proc. Yorkshire Geol. and Polytechnic Soc.,’ vol. 8, 1884, p. 375). They point out, quite justly, that 
the similarity in cortical structure is not by itself sufficient to prove identity. They go on, however, 
to state that the aaial structure of Astromyelon bears ‘‘by no means a close resemblance to that of 
Myriophylloides.”” This is a mistake, arising from the fact that, at that time, the primary wood of 
Myriophylloides had not been recognised. The structure of the original specimen of Myriophylloides is 
in fact identical with that of Astromyelon. For example, there is no real difference, except in age, 
between the root shown in “ Organization,” Part XII., Plate 28, fig. 2 (Myriophylloides), and that shown 
in Plate 27, fig. 3 (typical Astromyelon). On comparing our own fig. 1 (Myriophylloides) with fig. 2, 
which is from a typical Astromyelon, the essential identity of structure becomes obvious. The question 
of the identification of the smallest pithless specimens presents greater difficulties, and will be 
discussed in the text. 
