870 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
cases, however, the pith of these minute twigs is hollow.* It is impossible to doubt 
that in most cases, and apparently in all the larger branches, it soon became fistular 
in the living plant. This is shown both by the frequency and regularity of the casts 
of the medullary cavity, and by the very sharp limit of the outer persistent zone of 
the pith (see Plate 72, photographs 2 and 3, and Plate 77, fig. 8). 
In a few cases specimens are preserved which evidently represent the young 
condition of branches of considerable size. The most striking of these is the 
Culamopitus described and figured in 1871.+ Here there were about eighty vascular 
bundles, and though there had been some secondary formation of wood, its amount— 
about a dozen layers—is little in comparison with the dimensions of the stem. 
Other sections show from. fifteen to twenty-one vascular bundles, with little or no 
secondary tissue. Early stages of development of the more considerable branches, 
are, however, much rarer than mere twigs, as we should expect. 
In the arrangement of the vascular bundles, as seen in transverse section, we find 
the same variations as among species of Hquisetum at the present day. While the 
most common type is one with the bundles rather near together, the primary 
medullary ray having a width scarcely greater than that of the bundle itself, we 
sometimes find the bundles much more scattered, and separated from one another by 
rays more than twice their own width.t{ 
Similar differences are found, if, for example, we compare Hquisetum pratense with 
E. limosum. 
In the earliest stages observed, the vascular bundles, so far as their primary 
structure is concerned, appear to be already fully formed. In all cases the fascicular 
canal is open and well defined,§ and in all cases, too, there is a strand of thick-walled 
elements bordering on the outer side of the canal (see Plate 72, photograph 1, Plate 77, 
figs. 1 and 2). It is not always easy, in a strictly transverse section of a very young 
stem, to see which elements represent the xylem. When, however, the section is rather 
oblique, so that the markings on the lateral walls of the elements can be distinguished, 
it becomes evident that the thick-walled cells on the outer side of each canal are 
tracheze, and constitute the xylem-groups (see Plate 77, fig. 4). 
The most important question relating to the vascular bundle in its primary condition - 
concerns the nature of the canal which is always present on its inner margin. The 
presence of these canals, the internodal canals of previous memoirs, gives to the trans- 
verse sections an extraordinary resemblance in habit to corresponding sections of an 
* Plate 72, photograph 1, and Plate 77, fig. 2; also Witt1amson, loc cit., fig. 10. 
+ Witu1amson, ‘ Organization,” Part I., p. 488, Plate 25, figs. 19, 20, &e. 
¢ See Wittianson, “ Organization,” Part I., figs. 9, 14, 15, 20, 26. Onur own figures are all of the 
former type. 
§ We leave out of consideration some exceptional cases in which the canal is filled with a 
parenchymatous tissue. These will be discussed subsequently ; here it need only be said that in such 
instances the filling of the canal seems to have taken place long after its formation. 
