OF THE FOSSIL PLANTS OF THE COAL-MEASTJEES. 
17 
elongated. They vary considerably in their vertical length, those delineated in fig. 36, k 
being longer and narrower than they are in some other portions of the section of which 
the figure represents a small part ; but the tissue is always easily distinguished from 
that which it invests. 
Exogenous growth . — The difference between the sections figs. 24 & 25, in which we 
have merely the primary vascular bundle, and fig. 23 and other similar figures, in each 
of which what is obviously a corresponding bundle (c) is surrounded by six diverging 
wedges composed of radiating laminae of vessels separated by medullary rays, affords 
sufficient proof of the existence of exogenous growth in these plants. These wedges 
exhibit the same evidences of being the result of successive peripheral additions that 
we have already found in the instances of many other Carboniferous plants. This is 
especially the case with some, of part of one of which fig. 31 is a representation, enlarged 
130 diameters. It is a portion of a transverse section of the same specimen as is repre- 
sented in the figures 32, 33, Sc 34. We find at e , e the outer extremities of several vas- 
cular laminae, separated by the medullary rays whilst at g we have a portion of 
one of those crescentic masses of peculiar cellular tissue which I have already de- 
scribed as being located at the peripheral extremity (fig. 34, g) of each of the six primary 
vascular wedges. There is no mistaking the evidence that along the line (fig. 31, e', e') a 
conversion has been going on of the cells of this tissue into true vessels, destined to form 
outward extensions of the vascular laminae ( e , e). Oblique longitudinal sections show that 
the narrow cells ( e "), which are here being elongated in a direction parallel to the trans- 
verse diameters of the vessels with which they are in contact, are also elongated in the 
vertical direction, and are assuming a vascular aspect. Hence it becomes clear that the 
cellular tissue g is, as I have already stated, a special meristem tissue possessing pseudo- 
cambial properties, and the active instrument in producing the peripheral vascular 
growths. It thus appears that in these plants the pseudo-cambial layer never becomes 
a continuous ring as in most of the other exogenously developed plants which I have 
already described, but that it remains permanently in the state in which it exists in the 
first year’s growth of a Dicotyledonous Exogen, i. e. as a ring of detached masses, in 
which the interfascicular cambium has never been developed ; affording another in- 
stance, in the palaeozoic vegetable world, in which conditions that are temporary and 
transitional in ordinary exogens are rendered permanent ones. 
It is impossible to study this plant without being reminded of the Bignoniaceous 
Llianos of the Brazilian forests ; but though in these latter plants the woody axis is pri- 
marily divided into four distinct radiating wedges, and though these four wedges con- 
tinue to grow prominently in advance of the spaces occupied by the cortical tissues that 
separate them, yet even these latter spaces contain a true cambial layer, through the 
agency of which they become filled up with vascular tissues that gradually encroach 
peripherally upon the cortical structure ; and as they advance, they destroy the perfect 
form of the Maltese cross which transverse sections of these woody axes exhibit. Hence 
the resemblance between my fossil and true Bignoniaceous Exogens is more apparent 
MDCCCLXXVI. D 
