58 
Scott. — On a Palaeozoic Fern , the 
interpretation there is no * axillary 9 organ at all ; the branching is regarded 
as an unequal dichotomy, of which the main stem and the smaller (so-called 
5 axillary ’) shoot are the two limbs. The leaf associated with the smaller 
branch is the first appendage of that branch and not its subtending organ. 
This interpretation was mentioned by me in 1908 1 as a possible one ; 
I then regarded the data as insufficient to settle the question. Dr. Kidston 
(TO, pp. 451,454) has adopted this view ; he speaks of our ‘ undivided leaf- 
trace ’ as a ‘ branch and describes the petiole trace as an independent 
organ arising from the periphery of the ‘ branch ’ (i. e. branch-stele). 
The interpretation of the branching of Z . Grayi as a dichotomy is 
supported by the analogy of Z. corrugata , in which, to all appearance, 
a real and equal dichotomy of the stem and its stele occurs. 2 
After the investigation of the Shore specimen of Z . Grayi , I can 
no longer regard the question of the morphology of these organs as an open 
one. The evidence of that specimen seems to me decisive in favour of the 
view that the meristele, from its base upwards, is a leaf-trace. The peri- 
pheral loops are essential parts of the leaf- trace or foliar bundle ; they have 
nothing to do with the stem or its branches. Now we have seen that 
in the Shore specimen the peripheral loops are already fully differentiated 
where the meristele first becomes free from the stele, and even at a con- 
siderably lower level. It is impossible any longer to contend that an organ 
with these characteristic foliar structures can be of the nature of a branch. 
The general form of the meristele further supports the obvious leaf-trace 
interpretation. What is true of the Shore specimen must moreover hold 
good for the other specimens also. In the type specimens, as we have seen, 
leaf-traces of the same form as those of the Shore fossil occur. In the 
later Williamson specimen the form of the meristele is different, and 
the peripheral loops perhaps differentiated later ; they exist, however, 
where the meristele is still undivided ; the relation of the meristele to the 
main stele on the one hand and to the axillary stele and foliar bundle 
on the other is the same as in the Shore plant. 
It thus seems to be established that in Z. Grayi the meristeles given 
off from the arms of the stem- stele are leaf-traces, altogether homologous 
with those of Z. corrugata or Diplotabis Romeri. With the former plant 
especially, there is a close agreement in the mode of emission of the leaf- 
trace, if we allow for the difference in the form of the stele from which it is 
given off (P. Bertrand, ’ 09 , p. no; PI. XII, Figs. 87-9). 
1 Studies in Fossil Botany, 2nd ed., p. 318. 
2 This seems to be rare. I only know of one specimen (to be described fully in a forthcoming 
paper) which shows the branching in this species. Dr. Kidston’s citation (TO, p. 454) of the 
brief reference to the fact in my Studies (p. 318) might be understood to imply that I regard the 
leaf-traces in Z. corrugata as of the nature of a dichotomy. This is not, of course, the case ; the 
forking of the stem involves an equal division of the stele and has nothing to do with the well- 
known process of the emission of a leaf-trace. 
