498 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
This has been the case with our Arran plants, in most of which this bast-layer has 
increased with the general growth of the stem to a thickness of fully two inches. As 
in the stems referred to by Dr. Dawson, deep longitudinal furrows exist at its outer 
surface,—but these furrows are, as I have already shown, perfectly distinct from those 
of the true Sigillarice. They are but the fissures occasioned by the increase in the 
diameter of the stem, due to the growth of its more internal tissues. One fact is 
rendered further obvious from these specimens, viz., that this bast tissue increases in 
thickness along with the increase in the diameter of the other tissues forming the 
stem, but my specimens throw no additional light upon the physiological question 
discussed in my previous memoir (PartIX., p. 355), viz.: Whether the additions to the 
thickness of this bast layer are due to a plane of genetic activity on its external or its 
internal surface. Tangential sections of this bast layer exhibit no peculiarities in the 
arrangement of its prosenchymatous cells. 
I have already stated that I have obtained one cast or impression of what appears 
to have been the outer surface of the bark of one of these large stems. In it the leaf- 
scars are fairly preserved. They are elongated vertically, as shown in fig. 7, which 
represents the form of one of these scars three-fourths the natural size. There is a 
rounded central depression, a, in the cast, marking a corresponding elevation in the 
original bark, above and below which the cicatrix is prolonged upwards and downwards 
into a somewhat acuminate form; a slightly elevated sharp ridge, b, in the cast, mark¬ 
ing a corresponding vertical groove in the original cicatrix. These cicatrices are 
arranged in the usual oblique rows seen in Lepidodenclron, the centre of each scar 
being about five-eighths of an inch apart from its next diagonally-disposed neighbour. 
The rhomboidal and closely contiguous leaf-bases seen in the smaller branches are here 
separated widely apart from each other, besides being enlarged longitudinally. This 
fragment, however, may belong to a species distinct from the more abundant Arran 
examples. 
As already stated, the only fruits that have hitherto been found in the Arran beds 
are true Lepidostrobi. Of these Mr. Binney has figured three' 5 ' which he regards as 
distinct species, and all of which obviously contained, when living, both macrospores 
and microspores. In two of the specimens both kinds of spores are preserved, and in 
the third, which is only the basal part of the cone, the macrospores which it displays 
imply that microspores occupied its apex. The specimen already referred to, for which 
I am indebted to Professor Young and Mr. John 1 t oung, of Glasgow, is a true Lepi- 
dostrobus, but all its sporangia are empty. Of such fruits as Trigonocarpum and other 
forms, such as large and matured Gymnospermous trees might be expected to produce, 
not a trace has been discovered. We are thus led to the conclusion that these Arran 
trees are essentially Lepidodendroid. The large collection of specimens accumulated 
by Mr. Wunsch during a series of years was examined with the utmost care, to see if 
* ‘ Observations on the Structure of some Fossil Plants found in the Carboniferous Strata,’ Part II., 
plate xi. (Palseontographical Society.) 
