334 LYCOPODIALES 



interest for comparison with the Lepidodendroid fossils, in which a 

 peripheral protoxylem is found in the shoot, while a central protoxylem, 

 adjoining the medulla, is found in the Stigmarian trunks. 



Other species of Selaginella show further elaboration along distinct 

 lines. The simpler dorsiventral species, and even such radial species as 

 S. rupestris and oregana, show ribbon-like steles with marginal protoxylems, 

 upon which the leaf-traces are inserted. In the more complex cases the 

 axis becomes polystelic (S. inaequalifolia and Willdonovii}, or in some cases 

 solenostelic (rhizome of S. laevigatd), thus resembling similar vascular 

 complications seen in the stems of Ferns. These may be held to be 

 relatively late, and special developments from the non-medullated, mono- 

 stelic type : their origin shows parallelism of development rather than any 

 nearer relation with the similar structure seen in the Ferns. 



The near correspondence of the ancient Lepidodendron-type to that of 

 the modern Lycopodiales appears not only in their external form, but also 

 in their internal structure, though special modifications of type, different 

 from those of the modern forms, appear in accordance with the larger 

 dimensions so prevalent in the fossils. The similarity consists in the 

 presence of a single cylindrical stele, with a centripetal wood, and peripheral 

 protoxylem, in relation to which the leaf-traces are inserted with the 

 minimum of local disturbance. 



The general structure of one of the more simple types may be gathered 

 from Scott's figure of Lepidodendron Harcourtii (Fig. 174), which shows 

 (A) the relatively small proportion of the stele to the whole axis : (B) the 

 peripheral protoxylem, with its relation to the incoming leaf-traces, and the 

 uninterrupted metaxylem, not separated into strands : while centrally a large 

 pith is seen hollow in the middle. The steles of Lepidodendron varied in 

 structure towards the centre : in some cases such as the very ancient 

 Lepidodendron rhodumnense, Renault, and Lepidodendron saalfeldense, Solms, 

 from the Culm, there was a solid stele, without secondary thickening; 

 or, as in Lepidodendron Petticurensis, Kidston (Roy. Soc. Edin. Proc., 

 1906-7, p. 207), the solid xylem-core was surrounded by secondary wood. 

 But often, and especially in more recent forms, the xylem was medullated, 

 and in this they differ from modern Lycopods. It is obvious in some 

 cases that the pith originated by incomplete development of tissue originally 

 tracheidal: this is clearly indicated in Fig. 175. This drawing also shows 

 that outside the xylem came a narrow band, probably of phloem, which 

 is usually ill preserved, while in some cases there is evidence of an 

 endodermis, as in the present case. Thus, putting aside the larger size, 

 and the medullation which is its frequent concomitant, there is substantial 

 similarity in the structure of the stele to that of a simple Lycopodium, 

 or of Selaginella spinulosa at its distal region. 



A more striking concomitant of the larger growth was, however, the 

 secondary thickening represented in the majority of the known species of 

 Lepidodendron, though absent from some of the earliest. It was carried 



