October 13, 1911] 



SCIENCE 



467 



probability Isoetes, whieh Bower has aptly 

 summarized as like " a partially differen- 

 tiated Lepidostrohus seated upon a Lepi- 

 dodendroid base," we must inevitably con- 

 sider 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 pri- 

 mary feature in the morphology of both 

 these members of the Lyeopodiales. 



The somewhat considerable difference in 

 external appearance between the homolo- 

 gous 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 dif- 

 ferent fructification, we may unhesitatingly 

 compare with Isoetes as far as its root- 

 bearing axis is concerned. 



I was inclined at one time to seek an 

 analogy for the Stigmarian axis in that 

 interesting primitive structure, the proto- 

 corm of Phylloglossum, and of embryo 

 Lyeopods; but I now consider that the re- 

 semblances 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 quineuneial arrange- 

 ment and the apparent exogenous origin 

 of the roots borne on these underground 

 organs. Schimper, indeed, considered 

 these two features so characteristic of 

 foliar organs that he suggested that these 

 so-called "appendices" might possibly be 

 metamorphosed leaves. Not quite satisfied 

 with this view, Renault endeavored to es- 

 tablish 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. William- 

 son, however, clearly showed that the ap- 

 parent triarch arrangement was really due 

 to the presence at two angles of the meta- 

 xylem of the first tracheids of secondary 

 wood, and reasserted the existence of only 

 one type of appendicular organs, agreeing 

 so closely, both in structure 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 paleobotanist as 

 Solms Laubach, after a careful weighing 

 of all the available evidence, continued to 

 uphold Schimper 's view of the foliar na- 

 ture of these outgrowths, both in his 

 " Paleophytologie " 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 attention to their simi- 

 larity to the transfusion tissue of Lepido- 

 dendroid 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 mon- 

 arch root of the Isoetes type and a large 

 development 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 



