FILICINEM. 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 surrounded 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 laminae disposed m 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 

 laminoe 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 ficoides, 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 which 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. 1 77-1 81 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. So. 

 1873, pp. 152-156.] 



