590 MOSSES AND FERNS chap. 



ently no formation of secondary wood, in the majority of the 

 known species a zone of cambium arose outside the primary 

 wood, and from this were developed zones of secondary xylem 

 and phloem, precisely as in the Conifers and Dicotyledons. 

 The structure of the secondary wood, with the conspicuous 

 medullary rays, is strikingly like that of the wood of the Conif- 

 ers (Scott (i). Figs. 53, 56). ^ 



In addition to the secondary increase in thickness in the 

 stem due to the activity of the cambium, there was also a sec- 

 ondary thickening in the cortical region due to the formation 

 of a periderm, or cortical cambium. This mode of thickening 

 has been compared to that in Isoetes, and it also is not unlike 

 that in arborescent Monocotyledons, such as Dracmia and 

 Yucca. 



In SigiUaria, whose stem structures are seldom well pre- 

 served, there was in most cases a ring of separate vascular 

 bundles and a large central pith, and in the former respect the 

 typical SigiUaria stem is even more like that of the Conifers 

 than is that of Lepidodendron. 



In both Lepidodendron and SigiUaria the structure of the 

 leaves was more complicated than that of the living Lycopods, 

 and in certain respects they recall those of the Conifers (Scott 

 (i). pp. 148,. 204). 



The sporophylls of the Lepidodendracese were arranged in 

 cones or strobili, closely resembling those of their living rela- 

 tions. (Scott (i). Figs. 47, 48, 65). The strobili have been 

 described under the name of Lepidostrobns. The sporangia 

 were very much larger than those of any living Pteridophytes, 

 in Lepidostrohus Brownii reaching a length of two centimetres. 

 In their large size and. sessile position, they suggest the spo- 

 rangium of Isoetes, with which they agree also, according to 

 Bower (15) in the development of partial trabeculse. The 

 structure of the sporangia has in many cases been preserved 

 with wonderful perfection, and the spores themselves are often 

 encountered. In some species, e. g., L. Oldhamius, spores of 

 only one kind are known ; in others heterospory is very evident. 

 Whether the former type is really homosporous, or whether, as 

 yet, only microspores have been found, is not certain. 



Another type of lycopodiaceous cone has been found and has 

 received the name Spencerites (Scott (i). Fig. 71). The spo- 

 rangia in Spencerites were short-stalked, and evidently not very 



