ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 895 
the medullary tissue had disappeared,* the study of these plants underwent a radical 
change. The idea that the base of a large lateral branch adhered to the stem solely 
by a minute constricted neck, having been shown to be an impossible thing,t search 
had to be made for the true sustaining structures, and these were found in the strong 
zone of secondary xylem. 
The following series of photographs, taken from seven specimens of medullary casts¢ 
preserved in the Natural History Department of the British Museum, illustrate the 
structure of the branch, from its base upwards. Such casts, when perfect, invariably 
terminate at their lower extremity in a very narrow conical point, on which no traces 
of the vertical grooves and ribs, indicating the number and position of the vascular 
bundles, and the primary rays, are present. This point corresponds to what, in the 
following tables, is designated the proto-medulla of the branch, 2.¢c., the basal portion 
of its pith by which it was connected with that of the parent stem. 
These tables give the length and circumference of each internode, as well as the 
number of its woody wedges (vascular bundles); in all these respects an increase 
takes place in each successively higher internode, for a limited distance, as we ascend. 
Beyond the point where this increase is checked, which is usually within a few 
inches from the conical base of the pith of the branch, the latter is prolonged to a 
more or less considerable height, with very little further change in the features 
referred to. This fact was remarkably illustrated by the medullary cast of a stem, 
discovered by Mr. GzorcEe WI1p in the roof of a colliery under his superintendence 
near Ashton-under-Lyne. This specimen, which was a portion of the middle part of 
a Calamite, was 30 feet in length, but while the diameter of the pith at its lower 
end was 11°5 centims., at its upper extremity it had only increased to 15°2 centims., 
a very small increase in a stem of such a length, 
The photographs represent (with the exception of F) the basal portions of the 
specimens, and the accompanying tables give the measurements and number of 
vascular bundles in each internode, as above explained. 
* Wixiiamson, “ Organization,” Part I., 1871. 
+ Wixuamson, “ Organization,” Part IX.; see Plate 21, fig. 30. 
MDCCCXCIV.—B, 5 YY 
