454 Kids ton.—Note on the Petiole of Zygopteris Grayi , Will. 
and branch traces might of course be explained by the hypothesis that this 
basal portion is really a stem-structure on which the leaf is inserted, but this 
consideration, even if valid, will not apply to the structure of the petiole 
strand above its junction with the branch-strand. The whole of the pheno¬ 
mena, taken in conjunction with those of T. reniforme , appear to lend 
decided support to the interpretation of leaf-strand and stem-stele 
as primitive identical structures;’ 1 and again further: ‘ We do actually 
find, in the Botryopterideae and Hymenophyllaceae, the base of the leaf- 
trace exhibiting a structure which may be interpreted in this way.’ 2 In 
talking of Zygopteris it is stated : ‘ The leaf-trace departs as an isodiametric 
strand which divides tangentially as it passes through the cortex, the 
inner half becoming the stele of the axillary shoot, while the outer 
becomes the petiolar strand. The latter has at first the form of a tan¬ 
gentially extended band. . . .’ 3 It is evident then that Tansley regards 
the petiole and the remainder of the * axillary shoot ’ as two parts of a 
single organ, and this he believes to be cauline. 
Dr. Scott, however, from the study of the stem of Zygopteris cor - 
rugata where the leaf-traces are given off direct from the stem, without 
the occurrence of any ‘ axillary shoot ’, speaks of them as ‘ rather of the 
nature of a dichotomy ’, and gives the note of warning that ‘ this fact 
raises the question whether, as has been suggested, the apparent axillary 
branching of other species (of Zygopteris ) and of recent Hymenophyllaceae 
may not be a modified dichotomy, in which case the “ undivided leaf- 
trace ” would really be the stele of the small branch, and the ‘‘subtending” 
leaf would belong to this branch and not to the main axis. The data 
are insufficient to settle the question, and for the present it seems better 
to keep up the distinction between the two kinds of branching.’ 4 That 
the explanation of the nature of the ‘ axillary shoot ’ given here by 
Dr. Scott is the correct one is fully borne out by the development of 
the leaf-trace of Zygopteris Grayi, Will. 
In this species, no division of the so-called ‘ axillary shoot 5 takes 
place in the departure of the leaf-trace, but it arises from the periphery 
of that structure as an independent organ, and at no time partakes of the 
structure of the organ from which it arises. The so-called ‘ axillary shoot ’, 
till after the departure of the leaf-trace, retains a structure comparable with 
that possessed by the stem, and the only change which takes place in its 
structure is the possible loss of the pith, but this does not seem to occur 
until after the departure of the petiole, when it appears to be wholly formed 
of xylem, and assumes the condition of a solid protostele. As far as I have 
been able to observe, the petiole carries nothing of the branch with it, 
except it be a few tracheae from the two abaxial angles of the so-called 
1 Tansley, Evolution of Filicinean Vascular Structure, p. 33. 3 Tansley, 1 . c., p. 115. 
3 Ibid., p. 17. * Scott, Studies, vol. i, 2nd ed. p. 318. 
