496 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
at the other, whilst there are few in which the two sides are quite parallel*. But besides 
this peculiarity, the internode itself is unlike its neighbours, being only about half their 
length. Were this all, the internode might be regarded as an accidental anomaly; but 
when phenomena appear in regularly recurring series such an explanation is inapplicable. 
In Mr. Wilde’s fine specimen, of which fig. 29 represents a very small portion, every eighth 
internode exhibits these peculiarities. Similar appearances are seen in another specimen 
in the same collection, but here they appear in every fifth internode. I have as yet failed 
to correlate these appearances of the medullary cast with any known external features of 
Calamites, but that they have some special significance cannot be doubted ; they most 
probably indicate some specific features of the plant to which they belonged. 
Amongst the abundant stems of Calamites exhibiting the peculiarities upon which I 
have already dwelt, are others of a very different aspect. Plate XXVII. fig. 31 represents 
a well-marked example of a not uncommon type belonging to Mr. Nield, of Oldham. In 
the ordinary forms the casts are equilateral, being thickest towards the centre of each in- 
ternode, and having their smallest diameter at the constricted node ; but in the examples 
under consideration these proportions are reversed. The protuberance of the node is the 
most strongly marked on one side (fig. 31, i), whence a branch has been given off. The 
longitudinal surface-furrows are very strongly marked in the immediate vicinity of the 
node, but as they recede from it they become faint in outline and are at least doubled in 
number, indicating that, in the internodes (k), the number of the longitudinal woody 
wedges was mnltiplied, whilst the primary medullary rays became less conspicuous than 
at the node. On examining the prominent but concave phragma (Plate XXVII. fig. 32) 
or cicatrix left by the detached branch, we discover that there is a marked difference 
between the half of it above the nodal constriction and that below the line. In the former 
the surface-grooves and ridges bend uninterruptedly over the projecting margins of the 
cup-like cavity, and are prolonged to its centre ; but in the latter, though they enter the 
cup, they are arrested very near its margin, the rest of the lower half of the cavity being 
occupied by fractured matrix. These differences indicate corresponding ones in the rela- 
tions of the branch to each of the two internodes ; it springs from, and is organically 
connected with, the one below the node, whilst it is merely in contact with the one above, 
which it indents by the pressure occasioned by the growth of its truncated base. Plate 
XXVII. fig. 33 represents a similar specimen to the last, only it retains the branch belong- 
ing to the middle node in its normal position, which is rarely the case. In these two and 
some other allied specimens the branches spring from the main stems at definite angles, 
indicating some regularity in the phyllotaxis of the plant. The amount of the angle of 
divergence depends upon whether the growth has taken place from left to right, or from 
right to left. If the former, the angle of divergence has been 140°, which, according to 
Braun’s method of indication, would give the fractional symbol ; but if the latter, 
that angle would be 220°, giving the symbol |-g-. I have met with a few stems in which 
these branches were given off in verticils, three being planted on each node ; and I have 
* A similar condition is represented on one out of four internodes in Tab. 6, fig. 7 of Petzholdt’s “ Ueber 
C'alamiten und Steinkohlenbildung.” The plant figured is Calamites approximatus. 
