466 



SCIENCE 



[N. S. Vol. XXXIV. No. 876 



in the sections of Calamites and Sigillarias 

 of the presence of secondary wood had 

 -caused Brongniart to place these plants 

 among- conifers, owing to his belief that no 

 vascular cryptogams exhibited exogenous 

 growth in thickness. It required all Will- 

 iamson's eloquence and pugnacity to con- 

 vert both British and French paleobotan- 

 ists to his views, ultimately accepted with 

 such handsome acknowledgment by Grand' 

 Eury, one of his antagonists, in his "Geol- 

 ogie et Paleontologie du Bassin Houiller 

 du Gard. " 



It is curious that Grand' Eury refers in 

 his introduction to the discovery of traces 

 of secondary growth in OpMoglossum, and 

 not to that of Isoetes, a plant much more 

 nearly related, as we now believe, to the 

 Lepidodendraceffi, and the structure of 

 which had been so thoroughly investigated 

 by Hofmeister. "Williamson, it is true, 

 refers to the secondary growth in the stem 

 of Isoetes in his memoir on Stigmaria, but 

 compares it with the periderm-forming 

 cambium of that plant, and does not there- 

 fore recognize any agreement in the second- 

 ary growth of these two plants. 



Adopting von Mohl's interpretation of 

 the root-bearing base of the Isoetes plant as 

 a "caudex descendens," Williamson insti- 

 tuted a morphological comparison between 

 the latter and the branching Stigmaria, 

 and eame to the conclusion that they were 

 homologous structures, a view which, as we 

 heard at Sheffield, is supported by Dr. 

 Lang on the strength of a reexamination 

 of the anatomy of the stock of Isoetes. If 

 we do not accept Williamson's interpreta- 

 tion of the Stigmarian axis as a downward 

 prolongation of caulome nature, the ques- 

 tion remains open whether this under- 

 ground structure represented a leafless 

 modification of a normal leaf-bearing axis 

 as is known in the leafless rhizomes of 

 Neottia and other saprophytic plants, or 



whether the Stigmarian axes were morpho- 

 logical entities of peculiar character. 

 Grand' Bury, in comparing them with the 

 rhizomes of Psilotum, accepted the former 

 alternative and, apart from morphological 

 considerations, was led to this view by the 

 fact that he had observed aerial stems aris- 

 ing in many instances, as buds on the hori- 

 zontal branches of Stigmaria. Confirma- 

 tion of this mode of growth is still re- 

 quired, but it is quite conceivable that there 

 may have been a mode of vegetative repro- 

 duction in the Stigmaria analogous with 

 that of Ophioglossum.- 



The alternative interpretation of the 

 Stigmarian axes as special morphological 

 entities has received weighty support from 

 Scott and Bower, who consider them com- 

 parable to the rhizophores of Selaginella, 

 which, as is well known, may either be 

 root-bearers, or under certain circum- 

 stances become transformed into leafy 

 shoots. This peculiarity has led Goebel 

 to regard them as special members, some- 

 what intermediate between stems and roots. 

 But though they might therefore be re- 

 garded as of a primitive nature, the rhizo- 

 phores of the Selaginellaceffi seem such 

 specialized structures that I incline to 

 agree with Bower that, as far as their cor- 

 respondence with Selaginella is concerned, 

 the Stigmarian axes would agree most 

 closely with the basal knot formed on the 

 hypoootyl of Selaginella spinulosa. See- 

 ing, however, that the nearest living rep- 

 resentative of the Lepidodendraceffi is in all 



- It is of interest in this connection to note that 

 Potonie has recently put forward the suggestion 

 that many of these vertical outgrowths from the 

 more or less horizontal Stigmarian axes, some of 

 which, as figured' and described by Goldenberg, 

 taper off rapidly to a point, without any trace of 

 ramification, may be comparable with the conical 

 ' ' knees ' ' of Taxodium, and represent woody 

 pneumatophores so common in the swamp cypress 

 and other swamp-inhabiting trees. 



