552 TRANSACTIONS OF SECTION K. 



comparable to the rhizophores of Selaginella, which, as is well known, may either 

 be root-bearers, or under certain circumstances become transformed into leafy 

 shoots. This peculiarity has led Goebel to regard them as special members, 

 somewhat intermediate between stems and roots. But though they might there- 

 fore be regarded as of a primitive nature, the rhizophores of the Selaginellaceae 

 seem 6uch specialised structures that I incline to agree with Bower that, as far 

 as their correspondence to Selaginella is concerned, the Stigmarian axes would 

 agree most closely with the basal knot formed on the hypocotyl of Selaginella 

 spinulosa. Seeing, however, that the nearest living representative of the Lepido- 

 dendraceae is in all probability Isoetes, which Bower has aptly summarised as 

 like ' a partially differentiated Lepidostrobus seated upon a Lepidodendroid 

 base,' we must inevitably consider the root-bearing base of Isoetes as homologous 

 with the branching axes of Stigmaria, whatever their morphological nature may 

 have been, and perhaps we shall be on the safest ground if we consider them 

 both as different expressions of the continued growth of the lower region of the 

 plant, which appears to have been a primary feature in the morphology of both 

 these members of the Lycopodiales. 



The somewhat considerable difference in external appearance between the 

 homologous organs of these two plants may be considered bridged over by the 

 somewhat reduced axes of Stigmariopsis and by the still more contracted base of 

 the Mesozoic Pleuromoia, which, in spite of its very different fructification, we 

 may unhesitatingly compare with Isoetes as far as its root-bearing axis is con- 

 cerned. 



I was inclined at one time to seek an analogy for the Stigmarian axis in that 

 interesting primitive structure, the protocorm of Phylloglossum, and of embryo 

 Lycopods ; but I now consider that the resemblances are largely superficial, and 

 do not rest upon any satisfactory anatomical correspondence. 



One of the features which has caused some divergence of opinion in the past 

 as to the morphology of the Stigmarian axis has been the definite quincuncial 

 arrangement and the apparent exogenous origin of the roots borne on these under- 

 ground organs. Schimper, indeed, considered these two features so character- 

 istic of foliar organs that he suggested that these so-called ' appendices ' might 

 possibly be metamorphosed leaves. Not quite satisfied with this view, Eenault 

 endeavoured to establish the existence of two types of lateral organs on the 

 Stigmarian axis, true roots with a triarch arrangement of wood and root-like 

 leaves of monarch type. Williamson, however, clearly showed that the apparent 

 triarch arrangement was really due to the presence at two angles of the 

 metaxylem of the first tracheids of secondary wood, and reasserted the existence 

 of only one type of appendicular organs, agreeing so closely, both in struc- 

 ture and in their orientation to the axis, on which they were borne, with the 

 roots of Isoetes that it would be impossible to deny the root nature of the 

 Stigmarian ' appendices ' without applying the same treatment to the roots of 

 Isoetes. 



Still, so distinguished a palseobotanist as Solms Laubach, after a careful 

 weighing of all the available evidence, continued to uphold Schimper's view of the 

 foliar nature of these outgrowths, both in his Palaeophytologie and in his memoir 

 on Stigmariopsis, in which he stated that he was in complete agreement with 

 Grand ' Eury's conclusion : ' Que ces organes sont indistinctement des rhizomes 

 et que les Sigillaires n'avaient pas de racines reelles, ainsi que Psilotum.' Indeed, 

 in reviewing the account I gave of the occurrence of a special system of spiral 

 tracheids in the outer cortex of the Stigmarian rootlets, Count Solms drew atten- 

 tion to their similarity to the transfusion tissue of Lepidodendroid leaves, and 

 asserted that we have here a further indication of the former foliar nature 

 of these rootlets. Personally, I still adhere to the belief, expressed at the time, 

 that these peripheral cortical tracheids represent a special development required 

 by a plant with an aquatic monarch root of the Isoetes type and a large develop- 

 ment of aerial evaporating surface. The fact that the lateral outgrowths from 

 the Stigmarian axis have been generally considered to be exogenous is not a 

 valid argument against their root nature, as the same origin is ascribed to the 

 roots of Phylloglossum and to those produced on the rhizophores of Selaginella. 

 Probably, indeed, as Bower points out in his masterly exposition of the ' Origin 

 of a Land Flora,' in dealing with the Lycopodiales, ' the root in its inception 



