45 
Zygopteris Grayi of Williamson . 
It is to be noted that the internal tracheides are of the normal 
elongated form and not short, as in the stem of Diplolabis Romeri described 
by Dr. Gordon (’ll). The parenchyma accompanying the tracheides in 
the internal rays as well as in the central tissue consists of much-elongated 
thin-walled cells (PI. Ill, Fig. 3). 
The tracheides of the internal xylem-zone range from about 45 /x 
to 100 jut in diameter. The smallest, in the outer part of the zone, sometimes 
show a spiral thickening (PL III, Fig. 3). 
The peripheral or main zone of wood consists entirely of large scalari- 
form tracheides from 100 /x to 250 /x in diameter, the larger sizes predominat- 
ing, if we leave the ends of the arms out of consideration. In the latter 
region the structure varies greatly according to level, as will be seen 
on examining any of the Photographs 1-6, PI. I. A xylem-arm which has 
just given off a leaf-trace (arm 5, Phot. 5) shows practically no differentia- 
tion ; the tracheides at the distal end are scarcely smaller than the others. 
On the other hand, an arm which is on the point of giving off a trace 
already has the characteristic leaf-trace structure in its distal part, which is 
about to become detached (arm 5, Phot. 3, and arm 4, Phot. 6). This 
structure consists essentially in the presence of peripheral loops, one at each 
extremity of the bicornute arm, and of an island of internal xylem on the 
median line (PI. I, Phot. 8 ; PI. Ill, Fig. 1). The intermediate arms show 
the various stages of transition from the undifferentiated to the differentiated 
structure, as will be explained in describing the process of emission of the 
trace. 
Structure of the Leaf-trace. 
The structure of the undivided leaf-trace is shown in PI. I, Phot. 8, 
where it has just parted from the stele, PI. Ill, Fig. 1, where it has already 
entered the cortex, and PI. Ill, Fig. 2, at a point further out on its course. 
The trace passes out so gradually that it is seen practically in transverse 
section throughout. The changes in this part of its course consist in 
a slight broadening of the middle part of the trace and in a gradual lengthen- 
ing of the c antennae the name given by Dr. Paul Bertrand to the extensions 
of the xylem forming the lateral bars of the H-shaped foliar bundle in the 
old genus Zygopteris. At the lowest level (PI. I, Phot. 8) the antennae are 
scarcely present ; the peripheral loops simply occupy the truncated ends of 
the lunulate bundle, facing obliquely outwards (cf. Phots. 4 and 5). At the 
level shown in PI. Ill, Fig. i, the antennae already form recognizable protru- 
sions, especially at one end of the bundle ; at a still higher level (Fig. 2) they 
are quite prominent; it will be noticed that at the level shown in Fig. 2 the 
abaxial is longer than the adaxial antenna, contrary to the usual condition 
in Ankyropteris (cf. PI. V, Fig. 13). 
The peripheral loop consists of a double or triple band of small 
