66 AMERICAN FERN JOURNAL 
formed with a scanty and transitory supply of water. 
So far as all the vegetative functions of root and leaf 
are concerned, the ferns have developed capabilities for 
living in moderately dry places as well as in extremely 
arid situations in company with cacti and thorny leaf- 
less shrubs. Just one family of ferns, the Hymen- 
ophyllaceae, have retained or acquired characteristics 
which make abundant water just as essential to the 
every-day vegetative processes of their leaves as it is 
to the sperms at the critical moment of fertilization. 
A microscopic examination of the leaves of filmy ferns 
would reveal the nature of their water requirements. 
even if it were not possible to observe the plants in 
their natural habitats. Their delicate fronds are found 
to be only one cell in thickness. There is no upper °F 
lower epidermis. no central tissue traversed by water 
vessels and honeycombed by intercellular air spaces; 
and there are no stomata to connect internal air cavities 
with the outside. All these features of the normal 
aerial leaf are replaced by a structure as simple as that 
of the smaller algae. The frond of the filmy ferns }S 
of course, provided with vessels, and they branch to 
each pinnule, but there are often as many as fifteen 
cells between the vein and the margin of the pinnule. 
It is plain that such leaves depend little upon the water 
that is conveyed to them by the vessels and rely largely 
upon their direct contact with a surface film of water- 
There are relatively few localities in the world where 
the climatic conditions are favorable to the existence 
of plants which demand a sustained supply of atmos- 
pheric water. The mountainous tropical islands of the 
world offer these conditions, and so do the most moist 
elevations of continental mountains at low latitudes. 
During the course of three extended visits to Jamaic 
the writer has had an opportunity to observe and study 
the filmy ferns in one of the localities where they reach 
