678 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
various modifications of the letter H; (2) similar modifications of T ; (3) modifications 
of U ; and (4) of X. In many instances these primary bundles have secondary ones 
superadded. The arrangement of the fibro-vascular bundles in the stem or rhizome is 
that of a network constituting a hollow cylinder, prolonged through the entire stem. 
From each mesh of this cylinder there arise one, two, or more fibro vascular bundles, which 
pass outwards to form the petiolar bundles ; but, as M. Trecul has pointed out, many 
of these are abortive, because, generally speaking, some or even many of the meshes, 
especially in creeping rhizomatous stems, fail to develop corresponding leaves. Where 
leaves are developed we find great variations in the arrangements of the petiolar and 
radicular bundles. Sometimes both these are given off from the primary cylinder of 
the stem ; at others one or more bundles are thus given off to the petiole, and the 
radicular bundles spring from them as they pass through the cortical parenchyma of the 
stem or rhizome. Even where a single fibro-vascular bundle passes outwards to the 
petiole it frequently subdivides into a number of others, variously distributed over the 
transverse section of the petiole ; but even in this case we always find on the upper 
portion of the section two dominant ones, from which the secondary bundles going to 
the secondary petioles are almost exclusively given off. 
The bundles, however subdivided throughout the greater part of the length of the 
petiole, sooner or later reunite as they approach its upper extremity, where, generally 
speaking, they constitute a single bundle, grooved or indented on its upper surface, and 
which M. Trecul has designated gutter-shaped (“gouttiere vasculaire ”). It is, of 
course, important to bear in mind the occurrence of these morphological variations 
within the limits of the same frond in attempting to correlate differing sections of fossil 
forms. 
The structure and composition of the fibro-vascular bundle and the mode of orienta- 
tion of the secondary fibro-vascular bundles require to be attended to. I can confirm, 
from personal investigation, many of M. TrIscul’s observations on both these points. 
Each bundle almost invariably consists of a combination of scalariform or reticulated, 
annular and spiral vessels, the two latter being located in varying special positions in 
the bundle in different genera and species of ferns. The vascular portion of the bundle 
is very distinct from the investing fibrous one, and its section describes very definite 
figures. The secondary petiolar bundles spring from these primary ones in various ways 
in the different types. In many cases (e. g. Pteris hastata ) the upper extremity of each 
lateral arm of the vascular figure X becomes elongated in alternating order, and the 
enlarged portion becomes detached, to form the secondary bundle. In other cases 
each of these upper vascular arms terminates in a loop, the concavity of which is directed 
inwards, . When a secondary branch is about to be given off, this loop (“ crochet ” of 
Trecul) becomes elongated, g, i n Adiantum macrophyllum). Then a double 
vascular partition forms, cutting off its outer extremity, which now separates as a 
distinct vascular ringjC^p supplying the secondary pinnule or leaflet. This ring soon 
opens out at its upper surface, KJ, forming a gutter-shaped bundle; and in a few cases 
