218 Yapp. — Two Malayan ‘ Myrmecophilous* Fergus, 
of the stem is developed in the first instance as a special 
water reservoir 1 . The character of the tissue ; its remarkable 
similarity to the aqueous tissue of certain other xerophytes ; 
its position with regard to the leaf-cushions, which is such as 
would ensure an abundant supply of water to the young 
developing leaves ; the fact that during life it does undoubt- 
edly contain large quantities of water ; and finally, the state- 
ment made by Goebel 2 that a leaf attached to part of the 
stem remained fresh for a number of days in a warm dry 
room, afford, I think, sufficient evidence of this. 
At the same time, we have to explain the somewhat 
remarkable fact that this elaborate tissue has an extremely 
short life. In P. carnosum I have rarely found the aqueous 
tissue intact at a distance of 2 cm. from the growing point ; 
though in P. sinuosum Goebel 3 states that the living aqueous 
tissue is found at distances varying from 2 or 3 to 7 or more 
cm. from the apex of the stem. Karsten 4 , however, says that 
this is exceptional, the stem being usually hollow at a point 
1-2 cm. behind the apex. 
Now although the oldest parts of the rhizome in both 
Ferns are usually quite dead, the ordinary tissues of the stem 
have a much longer life than the aqueous tissue, so that the 
galleries do undoubtedly penetrate living parts of the plant. 
The question is whether these galleries, as such, fulfil any 
useful purpose, or whether they merely take the place of the 
aqueous tissue, when this tissue has fulfilled its function, and 
is now no longer required. It is possible that the latter may 
be the true explanation, but in view of the short life of the 
water tissue, it would seem to be hardly sufficient. 
There remains yet the third possibility, i. e. that these 
cavities may act as air-shafts, a function suggested by 
1 Goebel, loc. cit, p. 16, regards it as a true aqueous tissue, but Karsten (’ 95 ), 
p. 182, appears to think that the function of water storage is equally well fulfilled 
by the other tissues of the stem. He lays stress on the absence of the tissue in 
other Ferns, and its early degradation in cases where it exists. 
2 Goebel, loc. cit., p. 18. Of course the external cortical parenchyma might 
have contributed some at least of this supply of water to the leaf. 
3 Goebel, loc. cit., p. 18. i Karsten (’ 95 ), p. 182. 
