I 
55 
has a fit companion in that other which interprets the leaves of Discliidia 
as protectors of the roots, but does not tell us what purpose roots serve 
in such a plaee.^^ As a matter of fact, these plnnts are also myrmecoph- 
ilous, the loaves furnishing shelter for ants, and the antii furnishing food 
which the roots absorb. Discliidia is rarely without ants and rarely 
without a considerable amount of debris about tlie roots inside each leaf 
brought by them. There are other Asdepiadwcece, epi])1iytic withotit 
evident structural modifications, the roots of which are invariably in 
aerial ants' nests. 
In all these cases it is likely enough that the plant derives some 
organic as well as mineral food from its tenants. 
REPBOUUCTXVE STKUCTUBES. 
The principles underlying the adaptations of the reproductive struct- 
ures of ferns (sporophytes) are very simple. The sporaiigia Jinist be 
protected during their development against injury l)y desiccation or other- 
wise; the mature spores must dry thoroughly enoughly to be easily nnd 
well scattered ; and the drying of the spores must not involve too great a 
desiccation of the frond, for an insigniiicaiit nuijjber of Pliilippine ferns 
suffer an annual loss of their leaves. The structures found in fenis 
are a compromise between these rather antagonistic principles. 
Ferns almost always protect their sporangia, at the same time that 
they avoid interference with the illutiiiiiation of the assimilating organs, 
by restricting the former to the nether surface; our physiological ex- 
ceptions are Psomiocarpa and Stenosemia, the vegetative and reproductive 
fronds of W'hich are distinct, and Lecanoptcris, which may not be entirely 
dependent on photosynthesis for its organic food. 
For the sake of facile niitrition and to preserve the normal exercise 
of its functions by the nether epidermis, the sporangia of practically all 
ferns, the vegetative and reproductive fronds (or pinnae) of which are 
alike, are collected into sori. Most ferns protect these sori by means 
of indusia. At San Eamon, 60 per cent of all Polypodiaceie have indusia, 
the remaining 40 per cent including 13 members of the old genus Achros- 
tichum and a number formerly put into Gymnograynme, beside all those 
with w^ell-detined nude sori. In the indusiate list are included the 
Pteridem (not including ''Gymnogramme''), they having, bionomically, 
indusia as truly as any ferns do. Any full discussion of the forms and 
origin of indusia w^ould be supcrflous here, in view of the attention they 
have received as most important structures in taxonomy, but it is })er- 
tinent to the subject of this work to point out that their structure fits 
the local demands upon it. Thus^ it is leathery in the two strongly 
w 
^ Scott and Sargant, in Ann. of Bot. (1S93), 7: 243, suggest that the roots are 
to absorb water, tlmse in the inverted pitchers of />. rufflcsiana eondensing the 
water transpired hy tlie interior of the leaves. 
