82 Gwynne- Vaughan . — Observations on the 
middle line, with its convexity directed towards the base of 
the stem and its concavity towards the apex. 
The transverse section of the free petiole is approximately 
oval, and its vascular strand, which lies towards the centre, 
is curved into the form of a horseshoe with the concavity 
directed adaxially (Fig. 5). The limbs of the horseshoe 
are enlarged towards their extremities ; it varies in degree 
of curvature according to the level in the petiole at which 
the section is taken, the limbs being more widely separated 
and the whole strand less curved towards the top of the 
petiole. Towards the base of the petiole the ground-tissue 
is exactly like that of the stem, being entirely sclerenchy- 
matous, with the exception of two or three layers of paren- 
chyma immediately surrounding the vascular strand, and a 
few similar elements scattered amidst the more central 
sclerenchyma. However, at a point higher up, the brown 
sclerenchyma at the periphery of the petiole becomes changed 
into a hypodermal zone of elongated finely-pointed fibres 
with thick colourless walls and slit-like pits, while all the 
rest of the ground-tissue gradually becomes thin-walled and 
parenchymatous. The brown sclerenchyma persists longest 
within the concavity of the horseshoe, and along its flanks. 
At rare intervals, the dense sclerenchyma occupying the 
periphery of the petiole is interrupted by short tracts of 
loosely packed cells visible on the outside as short grey 
streaks. In longitudinal section it is seen that these tracts 
consist of elongated sausage-shaped cells, with rounded ends 
whose longitudinal walls are not in close contact all along 
their length, but separate from each other at intervals, so 
that a number of intercellular spaces of different sizes appear 
between them. These intercellular spaces give the tissue 
a distinctive appearance both in longitudinal and transverse 
section, although in the thickness of their walls and in other 
respects its cells closely resemble those of the rest of the 
ground-tissue. The few stomata that are to be found upon 
the petiole are localized above these tracts of tissue. The 
petiolar parenchyma in a very large number of Ferns is 
