14 
PROFESS OK W. 0. WILLIAMSON ON THE ORGANIZATION 
Each of the numerous specimens figured in this part of my memoir exhibits some 
morphological feature, important to be noted, in a better state of preservation than in 
the other examples. Hence, instead of describing each specimen separately, it will be 
convenient to examine successively the several structures constituting the plant, pro- 
ceeding from within outwards. Since it appears easy to identify the homologies of this 
plant with those of Lepidodendron and Ast er op] ly Hites, I shall revert to the letters of 
reference employed in the memoirs describing those plants, to represent what appears to 
me to be identical or closely related structures in each of these three types of organization. 
The central axis ( c ) in fig. 23 has a diameter of *04. In figs. 32, 33, & 34 its 
diameter is -024, the central tissues in these three sections having been destroyed. In 
fig. 26, which is a section from the same stem as the last three, but about half an inch 
higher up than fig. 34, it is rather larger ; but it has been subjected to a little lateral 
compression, giving it an oval instead of a rounded outline. In fig. 25 it is '028, and 
in fig. 24 it is -024. In fig. 37 it is ’008, whilst in fig. 38 it only consists of a few 
vessels. The entire diameter of the stem in fig. 23 is about 0T, whilst in figs. 32-34, 
had the bark been perfect, it would probably have been about 0T4. 
So far as I can ascertain, this central axis consists wholly of a bundle of vessels, which 
vary in size. The more conspicuous, larger ones in fig. 23 vary from ‘005 to '0025; in fig. 37 
the largest are not more than *0012, whilst in the young twig (fig. 25) the largest of them 
average about ’0025. It thus appears that they increase in size with the age of the 
stem, judging of that age from the degree of development of the exogenous cylinder. 
I have not succeeded in obtaining proof of the existence of any cellular tissue mingled 
with these vessels ; neither do they display any regularity in their arrangement ; 
irregular clusters of the larger vessels are connected by smaller ones, as represented in 
figs. 23 & 26. Those occupying the periphery of the bundle are generally uniformly 
smaller than is the case with those of the more central portion. In the two young 
twigs (figs. 24, c & 25, c) the entire bundle has a distinct hexagonal transverse section ; 
but this definite hexagonal contour is scarcely traceable in those examples which possess 
an exogenous zone. Longitudinal sections show that all the vessels of the central axis 
are, like those of the exogenous wedges, of the reticulated type (fig. 29, e). I have not 
yet discovered in any of my sections a single barred or spiral vessel. 
The Exogenous Zone.— As already stated, this consists of from four to six separate 
primary wedge-shaped masses of vessels, each of which is composed of numerous laminae 
in which the vessels are arranged in regular radiating series. In every one of what 
appear to be adult stems, such as figs. 23, 26, 32, 33, & 34 these wedges are six in 
number. The innermost vessels of each wedge are somewhat smaller than the more 
peripheral ones. In fig. 1 the largest peripheral vessels are about ‘0025 ; in fig. 32 
they are about ’0037 ; in fig. 37 the larger vessels are also about -0025. The number 
of the radiating lines of vessels seen in a transverse section of each primary wedge 
varies even in the same plant. Still more so when we compare figs. 37 & 38 with 
those of the more matured stems. The number of vessels composing each radiating 
