1883-84-] Edinburgh Naturalists' Field Club. 141 



dodendra alone, more than forty species have been constituted, dis- 

 tinguished by the beautiful markings on the stem; but as these 

 markings are found to vary on the same stem, the number is prob- 

 ably overestimated. The genus Lepidodendron has been described 

 as comprising "numerous large arborescent plants, which attain their 

 maximum in the Carboniferous period, but Avhich appear to commence 

 in the Upper Silurian, and are well represented in the Devonian. . . . 

 The bark is marked with numerous rhombic or oval scars, arranged 

 in quincunx order, and indicating the points where leaves were 

 formerly attached. The branches were covered with slender, pointed 

 leaves, closely crowded together; and the fructification was carried at 

 the ends of the branches in the form of cones or spikes. These cones 

 have generally been described under the name of Lepidostrohi ; an'd 

 they consist of a central axis, surrounded by imbricated scales or 

 bracts, each of which supports a sporangium or spore-case." ■*■ In 

 addition to this graphic description, it may be mentioned that these 

 Lepidostrohi or fruiting-organs are generally empty in the fossil state, 

 the spores having been shed. But we know for certain that many, if not 

 all, of them possessed both macrospores and microspores — the micro- 

 spores being produced in the upper sporangia of the spikes, the macro- 

 spores in the lower, or in the same relative position as in Selaginellse. 

 That the spores must have been present in vast numbers is evident 

 from the remnants which have been left us, and which yet form such 

 a mass that seams of coal several feet in thickness are almost wholly 

 composed of them. It is owing to their presence that shales and 

 several English coals owe their high degree of inflammability. Their 

 existence in coal was noticed fifty years ago, but their nature was 

 then unknown. The spores of some of our native Lycopods, as L. 

 clavatum and L. Selago, are also very inflammable. The yellow powder 

 which issues from their spore-cases is known in Britain as Lycopode or 

 Vegetable-brimstone, and in Germany as Lightning-meal and Witch- 

 meal. It was largely used in theatres for producing artificial lightning, 

 but modern science is providing electricity for this purpose. 



Another feature of the fossil Lepidodendra claiming attention is 

 the structure of the stem. "We have already seen that the stem of 

 Lycopodium is composed of a central cylinder of fibro-vascular bun- 

 dles, surrounded by a zone of cellular tissue, which becomes looser as 

 it proceeds outAvards to the epidermis. The Lepidodendron stem, in 

 the young stage, was almost identical in its structure. Indeed, if we 

 leave out of account, for a moment, the difference in size, the affinities 

 between the living and extinct forms are seen to be very close. But 

 as the extinct genus, while growing in the forest, became more mature, 

 and the tree required greater strength and stability, a curious modi- 

 fication of the structure took place. An outer cylinder of vascular 

 tissue was formed, the vessels being arranged in radiating wedges, 

 ^ Nicholson's 'Manual of Palteontology, ' vol. ii. p. 457. 



