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 byphse 

 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 Bromeliacese to settle on the green and still living leaves of Bromeliacese, 

 Orchidese, and Loranthacese, 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 stem 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 



