1925] JOHNSON—POLY PODIUM 243 
The epiphytic ferns and seed plants of temperate North America, 
such as Polypodium polypodioides, P. aureum, Vittaria lineata, 
Psilotum triquetrum Sw. (=P. nudum [L.| Griseb.), Tillandsia 
usneoides, and Epidendrum conopseum, and the eighteen others 
named by ScHIMPER, have each a more or less widespread distribu- 
tion in the American tropics, from whence they have probably 
migrated northward. The occurrence of Polypodium vulgare as 
an epiphyte in temperate North America, therefore, has a very 
interesting bearing on this question of the possible origin of extra- 
tropical epiphytes. For this fern, although distributed across the 
whole north temperate zone, in the New World from western Canada 
to Maine and south to Missouri and Georgia, and from Great 
Britain to Japan and southward into Northern Africa, is not known 
in the tropics, neither have fossils of it as yet been found there. 
We have no adequate evidence, therefore, that Polypodium vulgare 
acquired in the tropics the epiphytic habit which it assumes 
occasionally in Maryland, and more frequently in the damp forests 
of Portugal and Azores (SCHIMPER, 1888; CHRIST, 1910). The 
occurrence of this fern (or a closely similar polypody) in Cape 
Colony suggests that it may have crossed the equator by land, 
but of this there is no positive evidence, and this view seems 
negatived by the lack of fossils in equatorial Africa, and also by 
the absence of this polypody at the present day from the temperate 
highlands of the eastern tropics. From what is known of the 
habitats of Polypodium vulgare it seems most probable that this 
species is primarily a terrestrial plant of temperate forests. It 
probably entered North America from Eurasia via Alaska, and 
thence spread southward and eastward. It has acquired great 
hardiness while living on dry rocky ledges, often with a very scant 
water supply, and with no more soil than can collect in a few minute 
cracks of the rock. Thus this temperate zone polypody has come 
to be able to persist also in some shaded situations, on the very 
precarious supply of water and minerals to be found on the trunk of 
a rough barked tree. This is clearly true in spite of SCHIMPER’S 
somewhat too positive statement (1888, p. 152) that “in the less 
damp North American forests the first step, the migration of the 
terrestrial plants to the trees, is impossible, and herewith the origin 
