204 Yapp. — Two Malayan ‘ Myrmecophilous' Ferns. 
bands along practically the whole length of the petiole. 
Viewed from the exterior, these bands are lighter in colour 
than the rest of the petiole, owing to the absence of the dark- 
brown fibrous cells. Gwynne- Vaughan 1 suggests that this 
spongy tissue represents modified traces of a mesophyll de- 
current along the sides of the petiole, and remarks upon its 
significance with reference to Bower’s view that the leaf of the 
Ferns is a rachis or phyllopodium, fundamentally winged 
along its whole length 2 . 
Fig. 32 is a photograph of a longitudinal section through 
part of the apex of one of the conical leaf-cushions, showing 
the articulation between petiole and stem. The parenchyma 
of the leaf-cushion is seen to be deeply coloured with phloba- 
phene, while the tissues above, with the exception of the 
stelar sheath, are free from it. At the junction of petiole and 
leaf-cushion, there is a slight swelling, visible in the photo- 
graph as an outward bulging of the tissues. At this point 
a zone of rather small, isodiametric cells, several layers deep, 
stretches right across the leaf-base. Leaf-fall occurs by 
disarticulation along this zone of cells. 
As previously mentioned, a horizontal section through a 
leaf-cushion shows a horseshoe-shaped arrangement of bundles, 
passing upwards to the leaf, and partially surrounding the 
central gallery of the cushion. The concavity of the horse- 
shoe is adaxial, its free ends being formed by two bundles 
which are rather larger than the others. These two bundles, 
together with several smaller ones (often making a total of 
six), enter the petiole. Gradually the free ends of the horse- 
shoe 3 converge, and finally the two large bundles fuse with 
each other, though this does not occur in some cases until the 
summit of the petiole is nearly reached. The fate of the 
smaller bundles is not always the same. In the smaller 
leaves the rule is for the petiole, except at its base, to be 
1 Gwynne- Vaughan, loc. cit. 2 Bower (’ 84 ), p. 606. 
8 Gwynne- Vaughan, loc. cit., p. 95, points out that the vascular arrangement in 
the petioles of Ferns almost invariably takes the form of (in cross-section) a more 
or less modified horseshoe. 
