CONTENT OF AN EpipHytic FERN 101 
occasionally sees growing in the crotches of old trees must 
feed on bark detritus accumulated there, supplemented 
by a certain amount of dust, for such plants seem to 
be more frequent in and around settlements than far 
out in the woods. Falling leaves doubtless contribute 
a small quota too. 
The most widely distributed vascular epiphyte in 
the United States is the little resurrection fern, Poly- 
podium polypodioides (formerly known as P. incanum, 
and more recently as Marginaria polypodioides), which 
grows abundantly on living trees of various kinds from 
Virginia to Texas, mostly within 1000 feet of sea level. 
Like some other epiphytes, it is occasionally found also 
on non-calcareous rocks, not flat rocks on which water 
may stand and soil accumulate, but shaded cliffs and 
boulders (as in the case of its non-epiphytic northern re- 
lative, P. vulgare). There it has no connection with 
the soil (and is therefore inaccessible to earthworms, 
which seem to be detrimental to most evergreens), but 
probably gets all the nourishment it needs from decay- 
ing tree leaves that lodge around its roots. Another 
thing which its tree and rock habitats have in common 
is that they are rarely covered by snow or falling leaves; 
but whether that has any significance or not is not at 
present apparent.* 
As everyone who has seen this fern alive knows, it re- 
sponds readily to changes in atmospheric moisture. In 
dry weather the fronds shrivel up and look dead, but 
during and shortly after a rain, at any season of the year, 
they are fully expanded. The change from one con- 
dition to the other may take place in less than a day. 
Temperature seems to have little effect on it, except 
that the northern limit of the species may be deter- 
mined by the minimum temperature, or the duration 
*See Torreya 15: 30-31. 1915. 
