452 Kicist on.—Note on the Petiole of Zygopteris Grayi , Will. 
according to the state at which they have arrived in giving off branches ; 
that which has just given off a branch being shortest, while that from 
which one is just about to be given off is longest. If one of the stele arms 
in the latter condition be examined, it will be seen that its extremity 
is subtriangular in form, with the distal margin always straight, and termi¬ 
nating in two lateral horn-like points (Fig. i). Fig. 2 shows a branch just 
given off. The central area is occupied by a mixed pith, composed of 
parenchyma and small tracheae, and is a continuation of the mixed pith 
of the stem. 1 This is surrounded by a zone of tracheae, the adaxial portion 
being formed of one to three rows of moderately large tracheae, while the 
abaxial portion contains generally one—rarely two rows. The extremity 
of the lateral horns usually consists of a few small tracheae, which hold 
a special relation to the roots. In all essentials the ‘ branch ’ at this stage 
resembles the stem-stele with the exception of the five prominent angles. 
As the ‘ branch ’ travels outwards and upwards through the cortex 
it assumes a slightly more oval form, and the c horns ’ of small tracheae dis¬ 
appear, though it still contains a mixed pith, and possesses all the essential 
characters of its parent stem (Fig. 3). At this stage, the xylem of the 
abaxial side of the branch still usually possesses only a single row of 
tracheae. 
The beginning of the formation of the petiole trace is seen at Fig. 4. 
This stage immediately follows that described and seen at Fig. 3. The 
abaxial side of the branch shows a slight increase in the thickness of 
the xylem, and from each of the abaxial rounded angles tracheae, mostly of 
a size similar to those forming the branch-stele, begin to appear (Fig. 4, a\ a '). 
Traced higher up these two groups unite and form a bar placed 
tangentially to the axis of the branch as seen in Fig. 5, a } where it is still 
attached to the branch. Here the pith seems entirely to have disappeared 
from the branch-stele. The specimen shows a very small clear space in the 
centre of the axis, but this has much more the appearance of a break in the 
xylem than a space left by the decay of parenchyma. The specimen is, 
however, too imperfectly preserved for a definite conclusion on this point. 
I have not been able to observe on the specimens at my disposal 
the actual union of the two groups of tracheae which form the bar or 
the very early stages of the arm development, but from the examples given 
at Figs. 4 and 5, there can be little doubt as to the intermediate stages 
of development which succeed each other. 
At Fig. 6 the petiole trace has become quite free from the branch, and 
advanced some distance outwards as a free organ. The rows of tracheae 
on the abaxial side of Figs. 2, 3, and 4 have much increased in thickness, 
though the branch still shows an opening in the central area, but the manner 
in which fragments of walls of the surrounding tracheae project inwards 
1 See Kidston and Gwynne-Vaughan, Trans. Roy. Soc., Edin., vol. xlvii, p. 468, 1910. 
