298 
PROFESSOR W. C. WILLIAMSON ON THE ORGANISATION 
pages X One fact is undeniable, viz. : that certain portions of them, obviously stems 
or branches, possessed, when living, a true cambium which developed xylem and 
phloem in a normally exogenous manner. The same statement applies to two other 
plants described in previous memoirs, and which must be considered along with those 
just named. These are Lyginodendron Oldhamium and Heterangium Grievii; besides 
which close affinities appear to exist between the Lyginodendron and Rachiopteris 
cispera, also described in the Memoir, Part VI. 
Canals, precisely like those in the bark of Kaloxylon Hookeri, exist equally in the 
inner bark of Lyginodendron and of Rachiopteris aspera. The latter is unquestion¬ 
ably a rachis of a Fern ; whilst the former displays a wonderfully clear exogenous 
development of its xylem zone. I have more than once, in previous memoirs, 
suggested that the R, aspera was the petiole of the leaf of the Lyginodendron; and 
my friend, the Count de Solms, of Gottenburg, who has obtained numerous examples 
of these two plants from the Westphalian deposit at Pith Yollmond, has arrived 
independently at the same conclusion. If we are correct in this supposition, we have 
now, for the first time, in Lyginodendron Oldhamium , a Fern of which the stem or 
rachis exhibits a highly developed form of exogenous growth. This fact in some 
degree influences our interpretations of the two species of Heterangium , in both of 
which we find in the outermost bark the remarkable horizontal bands of dense 
sclerenchyma represented in Plate 22, fig. 11, of the present memoir, and in 
Plate 29, fig. 32, h', and Plate 31, figs. 45, 47, and 49 of the Memoir, 
Part IV., in which Heterangium Grievii was first described. This remarkable 
peculiarity in the structure of the outer cortex of these two plants has led to a 
careful search for anv fossil stems, with their foliage attached, in which a similar 
structure seemed to exist. Some months ago Mr. Ividson sent me some stems which 
he believed to belong to Sphenopteris elegans, the cortex of which displayed an 
exactly similar series of thickened horizontal parallel bands. Still more recently he 
received from my friend Professor von Weiss, of Berlin, and forwarded to me, a 
beautiful specimen of an exactly identical stem, attached to which are the unques¬ 
tionable pinnules of Sphenopteris elegans. So far as these internally structureless 
specimens affect the question, they suggest the possibility that both the species of 
Heterangium may also prove to be Ferns. At first sight, remembering the exogenous 
growth developing both xylem and phloem, as well as the discigerous tracheids of 
Heterangium Tiliceoides, this idea seems to be a most improbable one ; but it is no less 
probable than that Lyginodendron Oldhamium belongs to the group of Ferns, which 
latter conclusion has now made a near approach to certainty. 
The extraordinary vasculo-cellular structure of the medullary axis of the two 
Heterangiums finds its parallel in such examples of Kaloxylon Ilookeri as that repre¬ 
sented in Plate 24, fig. 27, of the present memoir. To see anything approaching 
this structure in other plants, we must go back to the somewhat similar central axis of 
Lepidodenclron Selaginoides (Memoir, Part XI., figs. 3, 5, and 10), which presents 
