OP THE EOSSIL PLANTS OE THE COAL-MEASURES.

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to whether the individual medullary rays, as seen in vertical tangential sections, consistedof one or more vertical series of cells. Brongniart having unhesitatingly accepted allWitham’s descriptions and figures, his conclusions respecting the British CarboniferousConifers are as untrustworthy as I have shown those of Witham to be. Notices ofvarious Carboniferous Conifers occur in Dr. Dawson’s numerous and valuable contri-butions to the history of Cryptogamic vegetation; but he more especially grappleswith the complicated definitions of previous authors in his “ Report on the Dossil Plantsof the Lower Carboniferous Millstone-Grit Formations of Canada.” In p. 15 ofthat report, published in 1873, after indicating the way in which various genera ofsupposed coniferous plants had been distributed and redistributed by preceding writers,he describes and figures the Dadoxylon antiquius, which species he had previouslydescribed in his ‘ Acadian Geology ; ’ a second species, also described in the ‘ AcadianGeology,’ appears under the name of Dadoxylon Acadianum ; a third is the D. annulatum;and a fourth is the D. materiarium, all of which are in like manner described in thewriter’s ‘Acadian Geology.’

In August, 1869, I published a brief memoir in the ‘Monthly MicroscopicalJournal,’ in which I endeavoured to distinguish those stems in which the vessels weremerely reticulate modifications of scalariform tissue, from those in which vessels orfibres exhibited true bordered pits or disks, such as are seen in the Coniferce and otherplants. In making an examination of recent stems, in connexion with that inquiry, myattention was arrested by some peculiarities in the vascular tissues of the Cycadeoe , afurther study of which led me to the following conclusion:—“ I have for some timebeen convinced that the discigerous vessels in Cycas revoluta, usually supposed to be ofa coniferous type, were in some measure modifications of scalariform tissue. I havenow found numerous vessels from the above plant which render the fact certain, sincethey exhibit discigerous tissue at one end of the vessel, whilst it becomes scalariformat the other extremity. My views on this point, when promulgated in private corre-spondence with some botanical friends, were at once rejected by them; but there is noreason for questioning their correctness. I was not aware, however, when I came tothis conclusion, that I had been anticipated by the late Mr. Don, in a paper which heread before the Linnean Society in 1840. The question is of some importance, sinceit affects the possibility of a glandular coniferous fibre being developed out of a reticu-lated one”*. The true nature, and still more the origin, of these disks seen on coni-ferous fibres was exceedingly obscure up to a very recent period. Even ITenfrey, inhis ‘ Elementary Course of Botany,’ speaks of “ the existence of a lenticular cavitybetween the contiguous outside walls of pitted cells, as in Coniferse (fig. 480),” and inthe figure referred to he represents this lenticular cavity as having walls of its own, inaddition to those of the two vessels between which it is located. It is obvious thatthese view's respecting the nature of the bordered disks made the passage of reticulatedinto discigerous fibres very difficult of apprehension. But the appearance of Sachs’s* Monthly Microscopical Journal, August 1, 1869, p. 69.

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