OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
681 
or reticulated ones, which a centripetal mode of growth produces subsequently and 
in more central portions of the bundle. The special positions of these spiral vessels 
vary in the different species of recent ferns, but their general arrangement is that just 
described. 
Immediately surrounding the vascular bundle we have the innermost of the cortical 
tissues ( g ), which varies much in thickness in different specimens. This is a delicate 
parenchyma ( g ), the cells of which are from ‘002 to *0025 in diameter, and which are 
disposed in interrupted vertical lines running parallel with the periphery of the bundle. 
These cells are nearly square, and must not be confounded with the prosenchymatous 
fibres which invest the vascular portions of each fibro-vascular bundle in living ferns. 
In the plant before us this fibrous sheath is wholly wanting. The line of demarcation 
between the innermost parenchyma and the middle one ( h ) is not very sharply defined, 
and yet we discern that we have passed from the one to the other from the rapid 
increase in the size of the cells, and in their diminished tendency to arrange themselves 
in vertical lines. This central parenchyma constitutes the greater portion of the bark. 
A large number of its component cells are fully *005 in diameter. The striking feature 
of this tissue consists in its containing a large number of oblong intercellular lacunae (i) 
of a dark colour, having their longer axes arranged radially in the horizontal direction. 
In Plate LI. fig. 3, two of these lacunae (i) are seen enlarged and, with the cells of the 
surrounding parenchyma (h), drawn with all possible accuracy. It will be observed 
that whilst most of the cells of the central parenchyma assume an ordinary arrange- 
ment, such of them, in the specimen figured, as immediately Surround each lacuna 
display a tendency to radiate from the lacuna as a centre ; but this, though a frequent 
arrangement, is not invariably the case. These lacunae vary in length from -002 to -008, 
the latter dimensions being the more frequent ones. 
The outermost bark (Jc) is seen, in longitudinal sections, to contain a large quantity 
of prosenchymatous tissue, intermingled with various modifications of parenchyma. The 
former is arranged in longitudinal bands more or less interrupted, and which correspond 
with the peripheral wedges ( Jc ) seen in fig. 1. Plate LI. fig. 5 represents an enlarged 
tangential section of this part of the plant. The narrow fibres of prosenchyma (Jc) are 
of great and variable length, and have a diameter of about *0028. The vertical inter- 
mediate bands of long, narrow, square -ended cells ( h ) are extreme modifications of 
parenchymatous forms, which gradually become shorter as we proceed towards the 
interior of the plant, and ultimately merge with those of the middle bark, of which they 
are merely prolongations separating the fibrous wedges as seen in fig. 1, h ' . The fibrous 
bands are obviously modifications of the sclerencliyma of authors, but which, instead of 
forming a continuous investing layer, as is so commonly the case in recent ferns, is here 
arranged in a network of longitudinally disposed bands with very long and narrow 
meshes. 
Though not appearing very prominently in the section represented by fig. 1, the outer- 
most bark projects at innumerable points (fig. 1, Jc") into small rounded or conical pro- 
