388 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
is occupied by a round and well-defined mass of vessels of two kinds, which the section, 
though made tangentially to the parent bark, has divided transversely, showing that the 
bundle passed out of the original stem at right angles to its vertical axis. The greater 
portion of the section consists of a cluster of vessels ( c ) not arranged in radiating series, 
larger ones occupying the centre and smaller ones the periphery of the cluster. In the 
former portion some of the vessels have almost disappeared ; but I have no doubt that 
originally there was an almost solid mass of them interspersed with some delicate cellular 
tissue. The periphery exhibits a narrow ring (d) of small vessels which are arranged in 
radiating series, the radiating lines being separated by medullary rays. Here we have 
obviously the first growth of an exogenous ligneous zone. 
Plate XXV. fig. 16 represents a second and yet more beautiful example, from one 
of the larger specimens to which reference has already been made. The cellular tissue 
(A), in which the branch-section is imbedded, is the central parenchyma of the bark of 
the parent stem, intersected tangentially : we have here, as before, the condensed ring 
of this latter tissue ( A ') enclosing the vacant space which I presume to have been occupied 
by a prolongation of the innermost bark-layer. The section of the branch, which is as 
perfect as possible, exhibits the same general arrangement of the tissues as that which 
appears in the previous example. The medullary axis is occupied, as in Lepidodendroid 
twigs, by a cylindrical bundle of vessels ; this, again, is surrounded by a well-developed 
ligneous zone, consisting of about twenty-three clearly defined, radiating vascular wedges, 
separated by equally distinct medullary rays. The exogenous development of this branch 
has advanced further before it emerged from the bark than in that just described. Tracing 
these branches inwards in order to ascertain their origin, we must return to Plate XXII. 
fig. 1, x, in the interior of which we discover a section similar in all respects to the last 
one, save that it has attained to a yet more remarkable degree of development before 
escaping from the pseudocambial and inner parenchymatous bark-layers. Plate XXVI. 
fig. 23 is a carefully drawn enlargement of a portion of this branch-section. Here, again, 
we have at c the compact mass of vessels forming the medullary axis, radiating from which 
we have a series of vascular wedges (A), which, in the perfect section, are nearly forty in 
number ; these, again, are separated by clearly defined medullary rays ( f ) of relatively 
large size, especially on the left and lower sides of the figure. As the section has traversed 
this part of the branch somewhat obliquely, we readily trace the mural arrangement of the 
cells constituting these rays ; whilst at their outer extremities we also see, with similar 
distinctness, how the cells merge in the equally delicate ones (g) of the pseudocambium of 
the parent axis (Plate XXII. fig. 1, g). Approaching nearer to the central axis of the stem, 
we find in Plate XXIV. fig. 11, x, another of these branches penetrating the ligneous 
zone ( e ). As already stated, this figure represents part of a tangential section of a ligneous 
cylinder made midway between the medulla and the bark ; but whilst the wood is inter- 
sected tangentially on the left-hand of the figure, the bark on the right is cut through 
almost radially — results due to the cylindrical form of the branch. The woody zone dis- 
plays the usual innumerable medullary rays separating the woody wedges; but at the 
