688 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
all question one of the most beautiful structures that I have hitherto obtained from the 
Carboniferous beds. Plate LV. fig. 28 is a transverse section of this petiole, and Plate 
LIV. fig. 29 a vertical one taken through its centre and along its longer axis. Its form, 
as seen in transverse sections, has been oval, with a major diameter of rather more than 
•5, and a minor one of about '87. It possesses, in its matured form, a double vascular 
bundle, a larger one {a) filling one focal portion of the ellipse, and a smaller one (a') 
occupying the other. Transverse sections of the former exhibit an hourglass-shape, and 
similar sections of the other display a crescentic one. The vessels composing the larger 
bundle (a) are remarkable for the regularity of their arrangement, comparative unifor- 
mity of size, and exquisite definiteness of outline. They are generally about '005 in 
diameter. At the peripheral extremity of this bundle (a") there is a deep longitudinal 
groove running down the side of the bundle, so deep that it is as nearly as possible 
enclosed by the vessels and converted into a canal*. The vessels in its immediate 
neighbourhood, and especially those forming the two crescentic horns embracing the 
groove, are much smaller than the more central ones of the bundle, being about '0016, 
and a few of them are even smaller still. These conditions are better shown in Plate 
LIV. fig. 30, which is a more enlarged representation of a vascular axis of this plant, the 
general form of which has been disturbed by pressure, but without any corresponding dis- 
turbance taking place in its individual vessels. The vessels composing the two horns en- 
closing the canal (a") are again seen to be of smaller size than those constituting the rest 
of the bundle, and a few similar ones exist on the opposite or central side of the groove. 
These distinctions are important, since on turning to longitudinal sections of these 
parts (Plate LIV. fig. 29) we find that these very small vessels are barred ones, whilst 
all the larger ones are reticulated. At the same time, as is shown by Plate LIV. 
figs. 31-34, there is a gradual transition from the one modification to the other. 
Figs. 31 & 32 represent two reticulated forms, the latter being the more common 
one ; fig. 33 is semireticulated, and fig. 34 simply barred. All the four figures are 
drawn to the same scale. At the opposite or more central end of the hour-glass 
bundle there is a second longitudinal groove (Plate LV. fig. 28, a"') ; but this is 
less deep, barely amounting to half a circle. A very small number of barred vessels 
occur in its vicinity, even the more minute ones being generally more or less reti- 
culated. On turning to the crescentic bundle («'), we find these conditions reversed. 
Generally speaking its vessels are smaller than those of the hour-glass bundle. A few 
of the largest are seen to be reticulated, especially as in fig. 33 ; but most of them are 
merely barred. I have been unable to detect any true spirals amongst them. In the 
longitudinal section (Plate LIV. fig. 29), a represents the central part of the hour-glass 
bundle ; a! the crescentic one ; a!' is the canal-like groove at the cortical end of the 
former bundle, with a few barred vessels on the right of it at the lower half of the 
figure; whilst at the upper half, where the section has just missed the groove, we find 
* I have obtained one specimen (Plate LY. fig. 35) in which the incurved crescentic margins absolutely 
meet, virtually converting this groove into the canal a". 
