Ridley.—Symbiosis of Ants and Plants . 467 
tions on the epiphytic flora and its growth. All trees do not bear epiphytes. 
On some of those with smooth bark or with longitudinally grooved bark 
epiphytes are seldom if ever to be found. Whether a tree does or does not 
bear epiphytes depends on the flow of rain down the branches and stem. 
Where in smooth-barked trees like Macaranga the rain flows quickly off, 
vegetable debris and spores cannot rest, and no epiphytes are borne. 
A notch in the bark of one of these may, however, retain a little soil, and 
epiphytes then usually appear. Lichens are abundant on these smooth- 
barked trees, and usually absent from rough-barked ones. Ficus Benjamina 
is a smooth-barked tree, but liable to cracks or other injuries, so that it 
carries epiphytes readily in parts. The boughs are covered with lichens, 
and where they are more or less vertical, with a strong slope, nothing more 
grows on the upper side where the great rush of rain-water takes place. 
On the sides where water more slowly trickles off, mosses and Algae start¬ 
ing from a crack in the bark commence growing. As they increase in 
growth they retain more and more of the debris, and may cover a consider¬ 
able patch. In so doing they kill out the lichens. 
When the patch is large enough ferns or phanerogamous plants appear. 
The roots of the orchids and creeping rhizomes of the ferns retain the 
vegetable debris washed down and blown by wind, and the plants increase 
until the bough may be covered with epiphytes. 
As soon as the orchids commence to grow, or even before, the ants 
begin to use the spot as a suitable one for their nests. The Pigeon-orchid 
(Dendrobium crumenatuni) is one very attractive to ants. It emits slender 
white roots so as to form a cage at the stem base, which is quickly occupied 
by a species of ant, a Dolichoderus . 
This ant brings a quantity of soil and piles it up, and fills in the 
spaces between the roots beneath which it makes the nest. The earth thus 
brought up supplies food to the plant and also serves to keep the roots cool 
and moist. 
The old nests as time goes on accumulate on the tree boughs till quite 
a quantity of soil is supplied to the roots of the various epiphytes. 
I do not find that all the great number of epiphytic plants on our 
trees are supplied in this way with nutriment, but undoubtedly it is one of 
the factors of the great development of epiphytes in the Eastern Tropics- 
Occasionally termites add a supply of soil to the epiphyte garden. In 
Arenga saccharifera and other palms of which the leaf-bases remain on the 
trunk, the termites tunnel out the leaf-bases and replace the destroyed tissue 
with mud, which remains in after the termites have left. Through this mud 
such plants as Davallia solida push their rhizomes and utilize the soil 
brought up. Ants too carry this soil further up the trees to form their nests 
beneath the orchids, &c. 
There are, however, no special modifications to induce the ants to nest 
H h 2 
