OcroDEU 19, 18.*3.: 



SCIENCE. 



b'■^b 



wliich one of the two diverging branches lias only 

 the centripetal cylinder of the Lepidodemlron, whilst 

 the other has begun to develop the secondary wood 

 of the Sigillaria.' 



The distinguished botanist of the Institut, Ph. van 

 Tieghem, has recently paid some attention to the 

 conclusions adopted by his three countrymen in this 

 controversy, and has made an important advance 

 upon those conclusions, in what 1 believe to be the 

 right direction. He recognizes the lycopodiaceous 

 character of the Sigillariae, and their close relations 

 to the Lepidodendra ;- and he also accepts my demon- 

 stration of the unipol.ir, and consequently lycopo- 

 diaceous, character of the fibro-vascular bundle of 

 the sligmarian rootlet, — a peculiarity of structure of 

 which M. Renault has hitherto denied the existence. 

 But along with these recognitions of the accuracy of 

 my conclusions, he gives fresh currency to several of 

 the old errors relating to parts of the subject to which 

 lie has not yet given personal attention. Thus he 

 considers that the Sigillariae, though closely allied 

 to the Lepidodendra, are distinguished from them by 

 possessing the power of developing the centrifugal or 

 exogenous zone of vascular tissue already referred 

 to. He characterizes the Lepidodendra as having 

 ' un seul bois centripete,' notwithstanding the absolute 

 demonstrations to the contrary contained in my Me- 

 moir xi. Dealing with the root of Sigillaria, which in 

 Great Britain, at least, is the well-known Stigmaria fi- 

 coides,following Renault, he designates it a ' rhizome,' 

 limiting the term 'root' to what we designate the 

 rootlets. He says, " Le rhizome des sigiUaires a la 

 m€me structure que la tige a^rienne, avec des bois 

 primaires tantot isole's ^ la peripheric de la moelle, 

 tantdt confluents an centre et en un ax plein; seule- 

 ment les fasceaux lib^ro-ligneux secondaires y sont 

 s^parcSs par de plus larges rayons," etc. 



Now, ,Stigmaria, being a root, and not a rhizome, 

 contains no representative of the primary wood of 

 the stem. This latter is, as even M. Brongniart so 

 correctly pointed out long ago, the representative of 

 the medullary sheath: and the fibro-vascular bundles 

 which it gives off are all foliar ones, as is the case 

 with the bundles given off by this sheath in all ex- 

 ogenous plants. But in the Lepidodendra and Sigil- 

 lariae, as in all living exogons, it is not prolonged 

 into the root. In the latter, as might be expected 

 a priori, we only find the secondary or exogenous 

 vascular zone. Having probably the largest collection 

 of sections of Stigmariae in the world, I speak un- 

 hesitatingly on these points. M. van Tieghem further 

 says, " La tige a^rienne part d'un rhizome rameux 

 trfes-d6\*loppe nomm^ Stigmaria, sur lequel s'insferent 

 & la fois de petites feuilles et des racines parfois 

 dichotomies." I have yet to see a solitary f.ict justi- 

 fying the statement that leaves are intermingled with 

 the rootlets of Stigmaria. The statement rests upon 

 an entire misinterpretation of sections of the fibro- 

 vascular bundles supplying those rootlets, and an 

 ignorance of the nature and positions of the rootlets 

 themselves. More than forty years have elapsed 

 since John Eddowcs Bowman first demonstrated that 



' Nfcmiiir xl.. pi. xlix., Bg. S. = Trnilc do botanlque, p. 304. 



the Stigmariae were true roots; and every subsequent 

 British student has confirmed Bowman's accurate 

 determination. 



M. Lesquereux informs me that his American ex- 

 periences have conviiiceil him that .Sigillaria is lyco- 

 podiaceous. Dr. Dawson has now progressed so far 

 in the same direction as to believe that there exists 

 ■A series of sigillarian forms which link the Lepido- 

 dendra on the one hand with the gymnospermous 

 exogens on the other. As an evolutionist, I am pre- 

 pared to accept the possibility that such links may 

 exist. They certainly do, so far as the union of 

 Lepidodendron with Sigillaria is concerned. I have 

 not yet seen any from the higher part of the chain 

 that are absolutely satisfactory to me, but Dr. Daw- 

 son thinks that he has found such. I may add, that 

 Schimper and the younger German school have always 

 associated Sigillaria with the Lyeopodiaceae; but 

 there are yet other points under discussion connected 

 with these fossil lycopods. 



M. Renault affirms that some forms of Halonia 

 are subterranean rhizomes, and the late Mr. Binncy 

 believed that Halouiae were the roots of Lepido- 

 dendron. I am not acquainted with a solitary fact 

 justifying either of these suppositions, and unhesi- 

 tatingly reject them. We have the clearest evi- 

 dence that some Haloniae, at least, are true terminal, 

 and, as I believe, strobilus-bearing, branches of vari- 

 ous lepidodendroid plants; and I see no reason 

 whatever for separating Halonia regularis from those 

 whose fruit-bearing character is absolutely deter- 

 mined. Its branches, like the others, are covered 

 throughout their entire circumference, and in the 

 most regularly symmetrical manner, with leaf-scars, 

 — a feature wholly incompatible with the idea of the 

 plant being either a root or a i-hizome. M. Renault 

 has been partly led astray in this matter by misinter- 

 preting a figure of a specimen published by the late 

 Mr. Binney. That specimen being now in the mu- 

 seum of Owens college, we are able to demonstrate 

 that it has none of the features which M. Renault 

 assigns to it. 



The large, round or oval, distichously arranged 

 scars of Ulodendron have long stimulated discussion 

 as to their nature. This, too, is now a well-under- 

 stood matter. Lindley and Hutton long ago sug- 

 gested that they were scars whence cones had been 

 detached, — a conclusion which was subsequently 

 sustained by Dr. Dawson and Schimper,^ and which 

 structural evidence led me also to support. The 

 matter was set at rest by Mr. d' Arcy Thompson's dis- 

 covery of specimens with the strobili in situ. Only 

 a small central part of the conspicuous cicatrix char- 

 acterizing the genus represented the area of organic 

 union of the cone to the stem. The greater part of 

 that cicatrix has been covered with foliage, which, 

 owing to the shortness of the cone-hearing branch, 

 was compressed by the base of the cone. The large 

 size of many of these biserial cicatrices on old stems 

 has been due to the considerable growth of the stem 

 subsequently to the fall of the cone. 



Our knowledge of the terminal branches of the 



' Mvmoir H., p. 222. 



