OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
679 
it again divides inferiorly, and forms two distinct fibro-vascular bundles, but which 
reunite as they approach the end of the pinnule or base of the leaflet. In some species, 
e. g. in Pteris umbrosa , the entire loop is detached to form the secondary bundle. 
In the larger number of ferns the secondary pinnules are not given off in pairs, 
hence the orientation of these secondary bundles alternates on opposite sides of the 
petiole ; in other cases, where the leaflets or pinnules are opposite, we have the same 
process of orientation occurring simultaneously on the two sides of the primary bundle. 
In dealing with fossil forms, we have to guard against confounding the two secondary 
bundles thus formed on opposite sides of the primary one with the subdivision of a single 
primary one into secondary ones as it emerges from the parent stem or rhizome to enter 
the petiole. This distinction is not always easy of recognition. 
There appears to be no doubt that the small outermost vessels of each arm of each 
bundle, whatever its form, are those first developed, and that the further growth is 
centripetal, the larger central vessels of the bundle being produced as the growth 
advances. Though these plants differ very widely in many respects from the Calamitean 
and Lycopodiaceous plants described in my previous memoirs, I believe that certain 
homologous relationships can be demonstrated to exist, enabling me to employ the 
same letters of reference to indicate corresponding organs and tissues, except that a has 
now been accidentally employed instead of c to represent the central vascular axis. 
The first of these plants which I have to describe is that to which formerly I proposed 
to assign the name of Pdraxylon. I have at length succeeded in connecting this petiole 
with its leaflets, which are either those of a Pecopteris or of a Sphenopteris. It has had 
the surface of its petioles covered with small warty projections, such as Brongniart has 
represented in several species of Pecopteris , especially in his P. platyracliis * * * § , in Sphe- 
nopteris Hoeninghausi f , and which appear in many recent ferns. 
Plate LI. fig. 1 represents a transverse section of the matured petiole, to which I 
originally gave the name of Pdraxylon%. It consists of a mass of parenchyma containing 
a bundle of barred vessels, and invested by a layer of sclerenchyma, which is not con- 
tinuous, but disposed in interrupted longitudinal bands. Each of these matured 
petioles is about a quarter of an inch in diameter. 
The central vascular bundle (a) is somewhat of the form of the letter H, with its 
upper and lower vertical arms diverging outwards from their central connecting band, 
and its lower ones much shortened. It consists, in fact, of a horizontal bar and two 
vertical ones, each bar having a variable mean thickness of from -016 to about T2§. It 
consists of transverse sections of numerous vessels arranged without any definite order, and 
varying in size, but rarely exceeding -0031 of an inch. I can discover no trace of a special 
* Vegetaux Fossiles, pi. 103. figs. 5-5 a. 
t Idem, pi. 52. This plant is also exceedingly well figured in the 1 Yorweltliche Pflanzen aus dem Stein - 
kohlengebirge der preussischen Eheinlande und Westphalens ’ of Dr. Cabl Justus Axdka, tab. iv. & v. 
X Proceedings of the Eoyal Society, No. 136. 
§ As in the previous memoirs, all these measurements are in decimal parts of an inch. 
