485 
so far as the Lepidodendroid and Sigillarian forms are concerned, our British forms all exhibit one 
type of internal organisation. In the very young state each twig has a central bundle of scalariform 
vessels sürrounded by a "bark," which usually exhibits an inner parenchymatous layer surrounded by 
a more prosenchymatous one, which is again invested by a second but more unequal parenchyma. 
This prosenchyma, as in Calamites, increases steadily in thickness as the growth of the stem 
advances, until it appears to constitute the chief tissue of the bark. Bundles of vessels given 
off by the central vascular axis proceed to each of the leaves. As the twig enlarges, the 
central axis almost invariably expands into a vascular cylinder, its interior becoming occupied 
by a cellular parenchyma of large size, and which now occupies the position and exhibits the 
appearance of a true medulla. The parenchyma of the leaves appears to be an extension of 
the outermost parenchyma of the bark. The above remarks appear to represent the common 
history of all the Lepidodendroid plants up to a certain stage of their growth. Beyond this stage 
their histories vary somewhat in the different groups. In some forms, e.g. those to which the 
Halonioe belong, the branches attain considerable dimensions without undergoing any great change 
in their internal organisation ; but in others a new development of vascular tissue invests the central 
cylinder at a period which seems to have varied in different species. This new growth takes place 
in successive layers, which are arranged in vertical lamince disposed in radiating planes separated by 
tracts of muriform parenchyma ; successive additions are made to the outer margins of the woody 
wedges previously formed through the agency of a pseudo-cambial layer of the innermost ' bark.' 
These exogenous growths continued until the woody zone attained to a great thickness in the larger 
trunks. These exogenous layers took no part in supplying the leaves with vessels. The foliar 
bundles invariably pass through them on their way from their source in the inner non-radiated 
vascular cylinder to the leaves. It being now admitted that Stigmaria was the general form of root 
of Lepidodendroid and Sigillarian types it is necessary to correlate its tissues with those of the 
aerial stem. It contains a "medulla" surrounded by a cylinder composed of radiating vascular 
laminse separated by cellular rays, and enclosed in a thick "bark." Large vascular bundles are given 
off from the vascular wedges to supply the rootlets. Thus the structure of the root differs from that 
of the aerial stem in two ways. (1) The inner vascular cylinder of the latter, characterised by the 
non-radiating arrangement of its vessels, by the absence of "cellular rays," and by the numerous 
foliar bundles which it gives off to the leaves, is altogether wanting in the former. (2) On the other 
hand, the exogenous zone of the stem is prolonged into the roots, retaining all its more important 
features. These however are modified in tv/o ways — ist, in the absence of small passages for the 
transmission of foliar bundles of vessels; and, 2nd, in their replacement by much larger spaces 
having a lenticular section, and through which large vascular bundles, directly derived by enlarging 
from the exogenous laminae themselves, pass outwards to the succulent rootlets. The rootlets of 
Stigmaria ßcoides, which equally belong to Sigillaria and to Lepidodendron, have a very remarkable 
internal organisation, identical with that which is characteristic of the roots of recent Lycopods, 
a fact w^hich affords additional confirmation of the close affinity of the Sigillarice and the Lepi- 
dodendra. That Lepidostrobi are the fruits of Lepidodendroid plants is certain. Equally so is it 
that many of the former produced microspores in the upper sporangia of each cone, and macro- 
spores in those occupying its basal end. The incalculable myriads of these macrospores found in 
many coals render it probable that a very large number of the Lepidostrobi possessed both kinds 
of spores ; indeed it is far from certain that any of them did otherwise. In the great majority 
of cases the sporangia of these fruits are shrivelled and empty, the spores having been shed ; and 
this renders it impossible to say what their original character was ^.'] 
^ [For the literature of the Carboniferous Lycopodiacese see Brongniart, Archives du Mus. 
d'Hist. Nat. vol. I, and Journ. Bot. vol. VII. pp. 3-8.— King, Edin. New. Phil. Journ. vol. XXXVI. 
— Hooker, Mem. Geol. Surv. vol. II. — Carruthers, Monthly Mic. Journ. vol. I. pp. 177-181 and 
225-227 ; Quart. Journ. Geol. Soc. vol. XXV. pp. 248-254. — Williamson, Phil. Trans, vol. CLXII. 
pp. 197-240, and Phil. Trans, vol. CLXXII. Part II, 1881. — Thiselton Dyer, Quart. Journ. Mic. Sc. 
1873, pp. 152-156.] 
