116 RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM. 
by thicker ropes or bands which run longitudinally down or encircle the trunk. 
These structures certainly serve as instruments of attachment, but at the same time 
they also absorb nutriment from the substratum, the decaying bark upon which the 
plant is epiphytic. In periods of drought the absorption of food by plants of this 
kind is, in general, interrupted and suspended. But when the rainy season 
commences and there is a long duration of wet weather, water trickling over the 
surface of boughs and trunks washes the bark, cleanses it as it were, and, falling 
lower and lower, brings down not only tiny loosened particles of bark but mineral 
and organic dust which has been blown into it by the wind; it dissolves all the 
soluble matter it finds on its way, and so reaches the roots, rhizoids, and hyphee 
which adhere to the bark, in the form of a solution of mineral and organic 
compounds, chiefly the latter. The trickling water is in some measure stopped by 
the projecting ridges of these adnate structures; here and there also it deposits 
particles mechanically suspended in it, and so it conveys to these curious epiphytes 
the requisite nourishment. 
In the same way, no doubt, epiphytes which grow upon other epiphytes are 
nourished. In more inclement regions, the green bark, stem, and, less frequently, 
the green leaves of the mistletoe are found to be beset by mosses and lichens; and, 
in the tropics it is a common phenomenon for mosses, liverworts, and even small 
kinds of Bromeliaceze to settle on the green and still living leaves of Bromeliaces, 
Orchidez, and Loranthacez, although they are certainly not properly parasitic, and 
only use their absorption cells for the purpose of clinging to the thick epidermis of 
the living leaves or stems which support them. The principal part of the liquid 
substances absorbed by these plants is conveyed to them by the rain-water that 
washes over the substratum. 
The species of plants also which have been mentioned as sometimes growing on 
smooth vertical faces of rock, though the bark of trees is their usual habitat, are 
able to obtain their food-materials in a similar way. If the summit of a cliff is 
covered by a continuous carpet of plants, or if ledges and terraces projecting 
somewhat from its face support sods of grass, tufts of moss, and various small 
kinds of bushes, it must inevitably happen when there is an abundant fall of rain 
that the water flowing down the declivity conveys with it organic compounds in 
solution. First the sods of grass and moss on the ledges and on the top of the cliff 
are wetted, then the humus, which is their substratum, becomes saturated, and such 
part of the water as cannot be retained by this humus, or does not percolate into 
the cracks and crevices of the rock, trickles down from the ledges and moistens the 
face of the rock as it soaks down to the bottom. A rocky declivity is thus washed 
in the same way as is the bark of trees, and small fragments of organic and 
inorganic bodies must of necessity be rinsed out and carried down by the trickling 
water, and then again be deposited in heaps where projecting obstacles are 
encountered. It is just in the tracks along which the water flows down steep 
rocks of the kind that the plants of which we have made mention are situated. 
Associated with the above are generally a number of other plants, for the most 
