Yapp . — Two Malayan ‘ My rmecophilous ’ Ferns. 195 
than are those composed of mere cellulose, even sulphuric 
acid requiring considerable time to act on them. This I have 
also found to be the case in the tissues of Polypodium car- 
nosum. But whilst increasing the resistance of tissues, the 
brown substance by no means renders them impervious to 
water, for even the living and apparently functional root-hairs 
of this Fern are deeply stained with it. 
The cell-walls of the ground-tissue are, when mature, con- 
siderably thickened. During the primary thickening of these 
walls, minute spindle-shaped thin places are left, which give 
the walls a faintly punctate appearance. Later on, broad 
thickening bands are laid down, which may join or cross each 
other so as to form an irregular network (Fig. 30). According 
to Baranetzki 1 , the thickening of parenchymatous cell-walls 
by threads or bands is very common, but is not always 
easily seen without the use of special staining methods. He 
remarks that lignification often makes it more obvious. This 
is also true of the brown colouring matter referred to above, 
as the thickening bands stand out with perfect distinctness 
on the brown walls of the old ground-tissue, while they are 
much more difficult to see before the tissue is coloured. 
A striking feature of the ground-tissue is the almost entire 
absence of intercellular spaces, both in the young and old 
parenchyma : at all events, I have been able to recognize only 
a very few, and these extremely minute ones in the thick- 
walled hypoderma. 
The vascular system. The structure of the steles is of the 
polypodiaceous type, and their arrangement resembles that 
found in other species of Polypodium with dorsi-ventral 
rhizomes 2 . The modifications in the course of the bundles 
are connected chiefly with the arrangement of the ant- 
galleries. 
In a transverse section of the stem, there is seen a single 
median ventrally-placed gallery, surrounded by a single ring 
of steles. These steles anastomose to form a network which 
has the shape of a modified cylinder. 
1 Baranetzki (’86), p. 135. 2 Cf. Klein (’ 81 ), pp. 335 et seq. 
