LEPIDODENDRON 139 



The inner zoneof cortex consisted of soft parenchyma, 

 usually badly preserved, while the more external region, 

 where the whole tissue was thick-walled, is perfect (Fig. 

 56, A). The cortex was covered on the exterior by the 

 crowded bases of the leaves, but the latter have often 

 been lost. In the outer cortex, a little below the leaf- 

 bases, periderm was formed at a rather early stage, by 

 the tangential division of the cortical cells. The 

 development of the periderm took place on both sides 

 of the initial layer, and was therefore partly centripetal 

 and partly centrifugal in direction. The leaf-bases 

 were thus separated by a zone of secondary cortex 

 from the inner tissues of the stem, but remained 

 attached to the outer surface of the bark, even on old 

 trunks. 



Very definite groups or strands of cells, no doubt 

 of the nature of internal glands, occur in the outer 

 cortex, just within the periderm. These secretory 

 organs are also present in various other Lepidodendreae, 

 as in Lepidophloios fuliginosus and in Lepidodendron 

 Wunschianum, where they are ranged in concentric 

 bands in the periderm. 



Except for the periderm, Lepidodendron Harcourtii, 

 as known to us at present, shows no sign of secondary 

 formations. It is, however, perfectly possible that 

 cambial activity may have started at the periphery of 

 the stele, in specimens still larger than any we possess. 

 That this may have been so, is rendered highly probable 

 by the analogy of another species, L. Wunschianum, 

 from the Lower Carboniferous strata in the Isle of 

 Arran. In this case secondary wood was formed in 

 great quantity, but it is never found except in stems 



