FROM THE LOWER COAL-MEASURES. 61 
Rachiopteris Williamsoni, which. until Mr. Seward distinguished it, had been included 
under Myeloxylon, agrees with the Suteliffia petioles in the general organization, in the 
concentric structure of the bundles, in the position of the protoxylem, and in the 
presence of xylem-parenchyma ; further, in the fact that the ground-tissue is traversed 
by sclerotic strands and secretory canals. The xylem-elements are the same in both, 
for tracheides with multiseriate pits are present in R. Williamsoni, though not specially 
mentioned by Mr. Seward. Even the state of preservation is similar, for in R. William- 
soni there is often a gap in the phloem next the protoxylem, just as is so often the case 
in the unilateral bundles of Sw£cliffia. Lastly, Mr. Seward’s fossil, like ours, is evidently 
derived from a rcof-nodule, as shown by the presence of Goniatite shells. There was 
thus some temptation to identify the two plants, and so to confirm the opinion 
expressed in 1899 that “ Rachiopteris Williamsoni was the petiole of some unknown 
Medullosean stem” (Scott, 1899, p. 117). Though the discovery of Sutcliffia insignis leaves 
little room for doubt that this view is correct, I do not think that the two fossils can 
be specifically identified. The sclerotic strands, grouped definitely round each vascular 
bundle, are a characteristic feature in Swécliffia, whereas in. Rachiopteris Williamsoni 
they are much less developed, and sometimes, in the case of the more internal bundles, 
wholly absent. The petiolar bundles of Sw¢cliffia are commouly more or less circular in 
transverse section ; in the neighbourhood of a fusion they may be elongated tangentially, 
i. e, in a plane at right angles to the axis of development of the xylem. In B. William- 
soni the more central bundles are often markedly elliptical, the long axis passing through 
the protoxylem, The difference is, of course, far from being an absolute one, but it is 
sufficient to give a somewhat different habit to the transverse sections. 
Lastly, the large elements regarded as sieve-tubes, which form a conspicuous feature 
in the bundles of Suécliffia, are by no means so well marked in R. Williamsoni, as an 
inspection of Mr. Seward’s figs. 7 & 8 will show. Similar elements no doubt occur, but 
they are smaller and much more scattered. The differences just pointed out are not, 
perhaps, very important, but they appear to be sufficient for specific distinction. 
Mr. Seward lays great stress on the presence of “ bundle-canals ” in R. Williamsoni, 
belonging to the peripheral part of the phloem, rather than to the fundamental tissue 
(Seward, 1899, p. 212). In many cases the “canal” appears in transverse section as 
a small group of cells, gradually separating from a central point, or in other cases 
showing distinct indications of disorganization (/. c. p. 213, figs. 11-13). Mr. Seward 
regards the different conditions observed as stages of a schizolysigenous process, leading 
to the formation of a secretory canal. I have occasionally detected similar structures in 
the phloem of the bundles in Swicliffia, and have also examined the conditions in 
Rachiopteris Williamsoni. I have some doubt as to the interpretation of these 
struetures as developing canals. 1t is not easy to see why such early stages of er 
ment (even if likely to be preserved) should be met with ina mature organ, where coin 
growth. I should rather incline to the view that these little 
is no question of secondary € = ge imperfectl 
nests of cells may represent strands of small sieve-tubes, more or less impertectly 
preserved. 
M 
SECOND SERIES.—BOTANY, VOL. VII. 
