OF THE FOSSIL PLANTS OF THE COAL-MEASUKES. 
223 
to whether the individual medullary rays, as seen in vertical tangential sections, consisted 
of one or more vertical series of cells. Brongniart having unhesitatingly accepted all 
Witham’s descriptions and figures, his conclusions respecting the British Carboniferous 
Conifers are as untrustworthy as I have shown those of Witham to be. Notices of 
various Carboniferous Conifers occur in Dr. Dawson’s numerous and valuable contri- 
butions to the history of Cryptogamic vegetation ; hut he more especially grapples 
with the complicated definitions of previous authors in his “Beport on the Fossil Plants 
of the Lower Carboniferous Millstone-Grit Formations of Canada.” In p. 15 of 
that report, published in 1873, after indicating the way in which various genera of 
supposed coniferous plants had beeji distributed and redistributed by preceding writers, 
he describes and figures the Dadoxylon antiquius , which species he had previously 
described in his ■ Acadian Geology ; ’ a second species, also described in the ‘ Acadian 
Geology,’ 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 the 
writer’s ‘Acadian Geology.’ 
In August, 1869, I published a brief memoir in the ‘ Monthly Microscopical 
Journal,’ in which I endeavoured to distinguish those stems in which the vessels were 
merely reticulate modifications of scalariform tissue, from those in which vessels or 
fibres exhibited true bordered pits or disks, such as are seen in the Coniferae and other 
plants. In making an examination of recent stems, in connexion with that inquiry, my 
attention was arrested by some peculiarities in. the vascular tissues of the Cycadece , a 
further study of which led me to the following conclusion : — “ I have for some time 
been convinced that the discigerous vessels in Cycas revoluta, usually supposed to be of 
a coniferous type, were in some measure modifications of scalariform tissue. I have 
now found numerous vessels from the above plant which render the fact certain, since 
they exhibit discigerous tissue at one end of the vessel, whilst it becomes scalariform 
at 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 no 
reason for questioning their correctness. I was not aware, however, when I came to 
this conclusion, that I had been anticipated by the late Mr. Don, in a paper which he 
read before the Linnean Society in 1840. The question is of some importance, since 
it 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 Henfrey, in 
his 4 Elementary Course of Botany,’ speaks of “ the existence of a lenticular cavity 
between the contiguous outside walls of pitted cells, as in Coniferae (fig. 480),” and in 
the figure referred to he represents this lenticular cavity as having walls of its own, in 
addition to those of the two vessels between which it is located. It is obvious that 
these view's respecting the nature of the bordered disks made the passage of reticulated 
into discigerous fibres very difficult of apprehension. But the appearance of Sachs’s 
* Monthly Microscopical Journal, August 1, 1869, p. 69. 
2 I 2 
