COMPARATIVE ANATOMY 337 



essentials the same construction x of the stele as in Lepidodendron, they 

 illustrate steps towards the breaking up of the primary wood of the 

 medullated stele into separate bundles. The details derived from various 

 Sigillarian fossils have lately been put together in stratigraphical sequence 

 by Kidston, 1 and his conclusion has already been quoted above (Chapter 

 XVIII., p. 230): he has shown a strong support for the view that the 

 condition with primary xylem forming a closed ring surrounding the large 

 medulla was the most primitive for Sigillaria : such a structure is found 

 in the more ancient specimens from the Lower Coal Measures (S. elongata, 

 Brongn., and 6". elegans, Brongn.) : those from the lower Permian, however, 

 (S. menardi, Brongn., and S. spinulosa, Rost. sp.) show the primary xylem 

 as a circle of separate bundles, though some of them may cohere laterally 

 in the last-named species. This indicates an evolutionary progression from 

 a concrete primary xylem to a condition where it is separated into strands. 

 In such forms the pith, being of relatively very large size, the primary 

 wood is reduced to a comparatively narrow investment round it, liable 

 as we have seen to be broken up into distinct strands. The secondary 

 tissues make their appearance, however, as in Lepidodendron ; there being 

 in Sigillaria a broad zone of secondary xylem, and a highly organised 

 periderm. It is thus seen that the later Sigillarias have departed further 

 in their structure from the simple protostele than other dendroid Lycopods, 

 for they show not only medullation, and a secondary thickening, but 

 breaking up of the primary xylem as well. 



It has been concluded above, on the basis of external comparison, 

 that the plant of Isoetes is like a partially differentiated Lepidostrobus 

 seated upon a Lepidodendroid base. The question will now be how far 

 its anatomy will countenance such an opinion. There has been some 

 confusion in the descriptions given by various investigators, owing doubtless 

 to the difficulty in decyphering a complex mass of tissues affected by 

 the reduction which follows on an aquatic habit. But this has been in 

 great measure cleared by Scott and Hill in their Memoir on Isoetes 

 hystrix, one of the few land-growing species. 2 Nevertheless the terrestrial 

 habit of this plant does not greatly affect its structure as compared with 

 other species, a circumstance which is held to point to the conclusion 

 that Isoetes is a genus which has long hovered about the limits of terrestrial 

 and aquatic life. The statement here given is based upon the Memoir 

 of Scott and Hill. 



The stele of the mature plant is not composed merely of the united 

 leaf-traces, but is best interpreted as a cauline structure, comparable to 

 that of the simpler monostelic Lycopods, but much shorter than is usual 

 in them. The crowded leaf-traces are inserted upon it, the stelar wood 

 serving to join up the xylem of the leaf-traces, but it does not belong to 

 one trace more than another, and in structure it differs from them. The 



1 Trans. Roy. Soc., Edin., vol. xvi., Part iii., No. 23. 

 -Ann. of Bot., vol. xiv., 1900, p. 413. 

 Y 



