4 * 
Zygopteris Grayi of Williamson. 
known fossil Ferns, but finding a close analogue in recent Hymenophyllaceae 
Where the trace of a foliage-leaf is to be given off the stelar arm broadens 
out at the end and acquires two prominent angles or horns (PI. II, Phots, n 
and 12). At the same time the large-celled xylem becomes continuous 
across the arm, cutting off the peripheral part of the internal xylem, which 
now forms an island (Phot. 12). When the trace detaches itself from the 
stele the line of separation passes through the large-celled xylem, which 
thus forms a closed ring in the leaf-trace, while it also remains closed in the 
arm from which the trace has parted (Phot. 13). Nothing of the nature of 
a leaf-gap is formed ; a real leaf-gap is of course out of the question, as 
there is no true pith. 
The leaf-trace, immediately on leaving the stele, as shown in William- 
son’s later specimen, is approximately triangular in transverse section with 
the base of the triangle outwards (PI. II, Phot. 13 ; PI. IV, Fig. 12). At 
the two abaxial angles are the groups of small tracheides already men- 
tioned, and in the middle of the strand is the island of internal xylem and 
parenchyma. The abaxial groups are destined for the foliar bundle, while 
the central island belongs to the axillary stele. 
The phloem closes round the outgoing trace as soon as it becomes 
detached, so that the bundle assumes concentric structure from the first 
(PI. IV, Fig. 12). It may be called the undivided trace, for it represents 
the common base of the foliar bundle and of the axillary stele. The 
separation between the two takes place where the trace is passing through 
the outer cortex, at a level where the base of the leaf is already prominent 
(PI. II, Phot. 11). The abaxial part of the common trace separates, to 
form the foliar bundle, the line of separation passing through the large- 
celled xylem, so that here, as in the departure of the trace from the stele, 
no gap is left in the strand behind. The foliar strand, as it becomes free, 
begins to assume the H form, but at first the shape is rather that of a very 
flat W with the base of the letter outwards, for the adaxial are longer than 
the abaxial ‘ antennae ’ and diverge from one another towards the axis 
(PI. II, Phot. 12; PI. V, Fig. 13). The phloem at once completes itself 
around both foliar bundle and axillary stele ; the latter at this level has an 
almost circular or slightly elliptical transverse section, the major axis in the 
latter case being tangential with reference to the parent stem (PL II, 
Phot. 10 ; PI. V, Fig. 13) ; the island of internal xylem persists throughout. 
A little higher up, the stele passes into the cylindrical axillary shoot, where 
it becomes free. I adhere to the terminology of Stenzel, reserving all 
morphological questions as to the nature of the leaf-trace and branch for 
future discussion (p. 57). 
The scale-leaves or aphlebiae, discovered by Renault in 1869 in his 
Zygopteris Brongniartii , and by Stenzel in Zygopteris scandens twenty 
years later (Stenzel, ’ 89 ), are seated both on the stem itself and on the base 
