II EQUATORIAL VEGETATION 255 
up the smooth bark of large trees, sending out roots as they 
ascend which clasp around the trunk. Some mount straight 
up, others wind round the supporting trunks, and their large, 
handsome, and often highly remarkable leaves, which spread 
out profusely all along the stem, render them one of the most 
striking forms of vegetation which adorn the damper and more 
luxuriant parts of the tropical forests of both hemispheres. 
Screw-pines 
These singular plants, constituting the family Pandanacez 
of botanists, are very abundant in many parts of the Hastern 
tropics, while they are comparatively scarce in America. 
They somewhat resemble Yuccas, but have larger leaves, 
which grow ina close spiral screw on the stem. Some are 
large and palm-like, and it is a curious sight to stand under 
these and look up at the huge vegetable screw formed by 
the bases of the long, drooping leaves. Some have slender 
branched trunks, which send out aerial roots; others are 
stemless, consisting of an immense spiral cluster of stiff leaves 
ten or twelve feet long and only two or three inches wide. 
They abound most in sandy islands, while the larger species 
grow in swampy forests. Their large-clustered fruits, some- 
thing like pine-apples, are often of a red colour; and their 
long, stiff leaves are of great use for covering boxes and for 
many other domestic uses. 
Orchids 
These interesting plants, so well known from the ardour 
with which they are cultivated on account of their beautiful 
and singular flowers, are pre-eminently tropical, and are 
probably more abundant in the mountains of the equatorial 
zone than in any other region. Here they are almost omni- 
present in some of their countless forms. They grow on the 
stems, in the forks or on the branches of trees ; they abound 
on fallen trunks; they spread over rocks, or hang down the 
face of precipices; while some, like our northern species, 
grow on the ground among grass and herbage. Some trees 
whose bark is especially well adapted for their support are 
crowded with them, and these form natural orchid-gardens. 
Some orchids are particularly fond of the decaying leaf-stalks 
