TREE FERNS OF NORTH AMERICA—MAXON. 465 
very existence is sharpest, and where, except for the acquisition of 
this trait or of some other to give them special advantage, their life 
would indeed be short. To meet the same conditions ferns of other 
families and of many tribes and genera have shown wonderful adapt- 
ability of different sorts, both in structure and change of habitat, 
two of the most common instances being the development of the 
climbing form and epiphytic habit of growth. In the intensely wet 
and heavily forested mountain region of Chiriqui, for example, prob- 
ably three-fourths of the ferns are epiphytic. To make best use of 
the arborescent habit, the growth of the tree-fern stem must be steady 
and of a permanent character; and we find that moist tropical con- 
ditions usually permit this, however slow may be the rate of develop- 
ment from season to season. 
Before proceeding, however, to a discussion of the widely different 
forms assumed by the many species, or of the more minute technical 
characters which serve to distinguish the genera and species, it may 
be well to note briefly the general factors which appear to control the 
distribution of tree ferns in North America. 
DISTRIBUTION AND HABITAT. 
As already indicated, tree ferns are characteristically inhabitants of 
wet, forested, tropical and subtropical regions and reach their best 
development in mountainous districts which are not subject to 
drought or pronounced seasonal change. In the Greater Antilles 
they are found mainly upon the northern slopes and summits of the 
higher mountains, as, for example, the Sierra Luquillo of Porto Rico 
and the Blue Mountains of Jamaica, where the cool, moisture-laden 
trade-winds from the northeast bring a constant and ample supply 
of moisture. The fern vegetation to the south of these mountains is 
more or less strongly xerophytic, both islands mentioned even having 
a semiarid region of cactus and scrub growth. Similar conditions 
were noted in the Sierra Maestra of Cuba. Here on the compara- 
tively dry southern slopes of the peak Torquino at 3,500 feet I 
found plants of Cyathea arborea, a species which in Jamaica and else- 
where in the West Indies rarely ascends to more than 2,000 feet. 
Associated with it were several polypodiaceous ferns which ordinarily 
are characteristic of the lowland forests and whose occurrence here 
so far above their usual limit is in all probability directly correlated 
with moisture supply. The southern coast in the lee of the Torquino 
is intensely hot and semiarid, with a dense ‘‘chaparal” formation 
(including cacti), wholly unsuitable not only to tree ferns but to a 
majority of ferns of other families, as well. 
Similar conditions upon a grander scale are observed upon the 
continent, the tree ferns being practically confined to the humid 
38734°—sm 1911——30 
