212 Yapp . — Two Malayan ‘ Myrrnecophilous' Ferns . 
section throughout the greater part of its length, though 
towards its summit it is more or less winged, the leaf-lamina 
being often slightly decurrent. The arrangement of tissues is 
the same as in P. carnosum. There are the same brown 
hypodermal fibrous cells, interrupted by the two lateral lines 
of spongy tissue, above which are the stomata, in this case 
even fewer in number than in the other Fern. The steles are 
usually two in number, a large elliptical one, adaxial in 
position, and a smaller circular one. 
The leaf-lamina, too, has a similar structure in both Ferns, 
but in P. sinnosnm the large cells under the upper epidermis 
appear to contain a certain number of chloroplasts. This is, 
no doubt, correlated with the difference in habitat of the two 
plants, P. sinuosum usually growing in somewhat shadier 
situations than P. carnosum. 
A diagrammatic drawing of a transverse section through 
a sorus is seen in Fig. 40. It is not so deeply sunken as the 
sorus of P. carnostim , and consequently the stalks of the 
sporangia are shorter ; the sporangia themselves, also, are not 
quite so large. Paraphyses, consisting of simple filaments 
of four or five cells, the terminal one rounded but not swollen, 
are fairly numerous. That part of the leaf-lamina which 
constitutes the bottom of the sorus-pits is thinner than is the 
case elsewhere. At the edges of the sorus-pits the cells of 
the epidermis are considerably deeper than is usually the case 
(a. Fig. 40). 
Biological Considerations. 
Under this heading may be discussed the curious reflexed 
position of the sori in Polypodium carnosum , and the meaning 
to be attached to the galleries which occupy such extensive 
tracts in the tissues of the stem in both Ferns. With the 
latter point is connected the question of the relations existing 
between the ants and their hosts. 
The reflexing of the sori of P. carnosum upon the upper 
surface of the leaf is a phenomenon in all probability nearly 
unique in Ferns, the only other cases of which I am aware 
