878 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
obviously formed part of the primary structure. All the preparations which we have, 
showing the insertion of branches, are sections of comparatively advanced stems, in 
which the secondary growth in thickness has already made considerable progress. 
Hence the structure of the branch-bases is much complicated, owing to changes 
connected with the formation of secondary wood, and until the latter has been 
studied in detail, the phenomena presented by the branches cannot be understood. 
THE SECONDARY TISSUES. 
So far as the first origin of the secondary tissues is concerned, the type of develop- 
ment in Calamites always corresponds to the simplest of the various modifications 
with which we are acquainted in recent Dicotyledons and Gymnosperms.* We have 
only to distinguish between fascicular aud interfascicular tissue, the former including 
all the tissue developed within the limits of each primary bundle, while the latter 
arises by the division of cells belonging to the primary medullary rays. In no case is 
there any preliminary formation of intermediate cauline strands, as so frequently 
happens among Dicotyledons. The secondary thickening of Calamites, in fact, so 
far as concerns its earlier stages, is of diagrammatic simplicity (see Plate 72, photo- 
graphs 1, 2, and 3; Plate 77, figs. 1, 2, and 3). 
As soon, however, as the zone of secondary tissue begins to attain any considerable 
thickness, variations make their appearance, depending chiefly on the degree in which 
the fascicular and interfascicular strands remain distinct in the secondary region, or in 
other words, on the part played by the principal medullary rayst during secondary 
growth, 
Among the specimens examined by us we find four distinct types as regards this 
point. These distinctions are, of course, purely anatomical, and may have little or no 
systematic value. We may group the various forms as follows :— 
A. Principal rays remain parenchymatous throughout the whole thickness of the 
secondary wood. 
As sub-divisions of this type we have : 
A 1. The ray maintains about the same thickness throughout. 
A 2. It becomes narrowed towards the exterior by the greater tan- 
gential growth of the fascicular wood. 
B, The principal rays disappear, as such, towards the exterior, owing to the 
formation of interfascicular wood. 
* See Ds Bary, ‘Comparative Anatomy of Phanerogams and Ferns,’ English Translation, p. 455, &e. 
+ We propose to limit the term primary ray to the truly primary interfascicular tissue. When this 
is prolonged into the secondary tissues by the cambium, we speak of it as a principal ray; the term 
secondary ray is applicd, as usual, to those intermediate rays, which are entirely of cambial origin, 
