Yapp . — Two Malayan ‘ Myrmecophilous ’ Ferns . 205 
provided with only two steles (Fig. 38). Of these, the larger, 
which is adaxial in position, is, at least in some cases, formed 
by the fusion of the two large bundles and two of the smaller 
ones ; the other is composed of the remaining two small 
bundles. In the larger leaves, however, there is not such 
complete fusion of the steles. The smaller ones anastomose 
or separate at intervals, but still retain to some extent their 
individuality. 
Lamina . The mesophyll, as seen in a transverse section of 
a leaf-segment, is differentiated into two parts. The first con- 
sists of a single layer of large cells, lying immediately beneath 
the upper epidermis (a. Fig. 51). At the margin of the leaf 
this layer may be doubled, its cells at the same time becoming 
smaller. On the under side it passes into the spongy meso- 
phyll. Viewed in optical section, by means of clearing the 
lamina with Eau de Javelle and staining, this layer is seen to 
possess no intercellular spaces. Its cells have more regular 
outlines than those of the ordinary mesophyll. They contain 
no chlorophyll when mature, and little or none even in the 
younger stages. This tissue may perhaps be regarded as 
a hypodermal water reservoir, similar to that found in the 
leaves of certain Orchidaceae, Velloziaceae, & c. 1 The bulk 
of the mesophyll, however, consists of the usual spongy tissue, 
composed of branched cells with irregular outlines and many, 
though comparatively small, intercellular spaces. 
The epidermis has a fairly thick cuticle. Its cells, except 
the guard-cells, apparently contain no chlorophyll. Stomata 
are numerous on the lower surface of the leaf, but are absent 
on the upper. The guard-cells are on the level of the 
ordinary epidermal cells and are not sunken, as is so often 
the case in the leaves of xerophytes. Their development 
corresponds to that described by De Bary 2 for various 
other Ferns. Successive stages in the formation of young 
stomata are shown in Fig. 49, a , , b , c } d i and a mature stoma 
in Fig. 50. 
The general course of the veins in a leaf-segment has been 
1 Cf. Warming (’96), p. 201. 2 De Bary (’84), p. 41. 
