538 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
the symmetrical boundary or endoderm, enclosing a sheath of endophloeum that has 
invested the axis, but which has disappeared. At c, c, we have a pair of vascular 
bundles passing upwards and outwards, also enclosed in a supposed phloem boundary, 
and at d, d, is a second similar pair which have entered the coarse parenchyma of the 
outer bark, but which are immediately surrounded by a parenchymatous zone, e, of a 
much more delicate texture. At /, on the opposite side of the section, we have a 
single bundle passing outwards, which from its symmetrical form and central position 
seems to correspond with the united bundles of each of the two opposite pairs. Here 
again the bundle, f is surrounded by thin-walled parenchyma. 
The greater portion of the tissues of the middle and inner bark have disappeared. 
Here and there we obtain faint glimpses of them, indicating that they consisted, as is 
usually the case in these ferns, of delicate, thin-walled parenchyma. 
On turning to vertical sections of this plant, we find that they agree with similar 
ones of the other species of the Zygopteroid group of fern petioles. Fig. 91 exhibits a 
vertical section through the central part, a, of the vascular axis, enlarged 56 dia¬ 
meters. Its vessels, a, are now seen to be of the most perfect scalariform type that I 
have hitherto met with amongst these carboniferous ferns. Immediately external to this 
vascular axis is a thin investment of three or four layers of very long and narrow, 
thin-walled, square-ended cells, b, b, the extreme delicacy of which makes them almost 
invisible save under high magnifying powers. Vertical sections through the transverse 
bai's, a, of the vascular axis, show that whilst the greater number of its vessels are of 
the same type as those of the single central bar, d, the outermost ones, some of which 
become detached to form the foliar (?) bundles, c, d, and f approach more closely to 
spiral vessels. The foliar bundles themselves consist wholly of vessels of the spiral 
type, enclosed in a distinct investment of delicate, oblong, square-ended cells. The 
outer bark, h, is seen to consist in the vertical section, of vertically disposed rows of 
cubical parenchymatous cells of large size and coarse texture. 
The orientation of the supposed foliar bundles in symmetrical pairs is a curious 
feature in several of these Zygopteroid forms. I have already described its occurrence 
in Rachiopteris duplex and R. Lacattii. The peculiar form which the transverse section 
of the vascular bundle of the specimen now described exhibits, so closely resembles 
that of two severally inverted Greek capital letters, that the stem may be provisionally 
recognised as Rachiopteris di-upsilon. 
Index to Figures. 
PLATE 21. 
Fig. 82. Transverse section of the central axis of a Lepidostrobus from Halifax. 
Enlarged 16 diameters, a. Central vascular cylinder. b. Cellular 
investments of the vascular bundles going to the bracts, c, c. Portions 
