CHAP. XXV SYMBIOSIS OF PLANTS WITH ONE ANOTHER 89 



with peculiar absorbing-hairs.^ Others (Orchidaceae, Araceae) have 

 aerial roots with a special velamen adapted to absorb water ; yet others, 

 for example Tillandsia bulbosa, have their leaves so constructed as to 

 facilitate the retention of water among them ; others, again, possess two 

 kinds of leaves, some of which, as ' pocket-leaves \^ are pressed closely 

 against the substratum so that water is held by capillarity between them 

 and the supporting-stem, or is actually taken up by them, as G. Karsten 

 suggests in the case of the fern Teratophyllum aculeatum. Epiphytes 

 are much exposed to desiccation. Against this, certain species (algae, 

 lichens, and mosses) have no evident protection ; they can endure without 

 injury existence in a dry condition for a long period, and awaken into 

 life again at the first fall of rain or dew. But others have fashioned for 

 themselves water-receptacles of various kinds : aqueous tissue in leaves 

 and stems (in Orchidaceae, Peperomia, and others), water-storing cells 

 in leaves (in Orchidaceae and others), urn-shaped water-bags or con- 

 cavities of other shapes (as in liverworts,^ Dischidia, and others). 



Food-material is obtained by epiphytes as follows : Carbon is taken 

 from the air, as all epiphytes are photophilous and evergreen ; some also 

 accumulate humous and mineral bodies among their roots or with the 

 aid of specialized leaves ('pocket-leaves', 'mantle-leaves'), as in the 

 cases of ferns such as Asplenium Nidus, Polypodium quercifohum, and 

 Platycerium alcicorne.^ 



The construction of the shoot and the whole architecture of the 

 epiphyte varies widely. Some species, like Tillandsia usneoides, are 

 rootless, while the vegetative organs of others, such as the orchid Poly- 

 rrhiza (Aeranthus) funalis, consist almost entirely of green roots. With 

 Schimper ^ we may divide epiphytes into four groups : 



1. Those that find their nutriment on the cortex of their support. 



2. Those that send aerial roots into the soil. 



3. Those that collect moist humus within the large interwoven mass 

 of roots, and, in some cases, behind or between their leaves. 



4. Those whose leaves take over the functions of roots and absorb 

 water and nutritive salts.^ 



Epiphytes have many structural features in common with terrestrial 

 xerophytes, for, like these, they must be adapted to endure prolonged 

 drought. They form in fact a group of xerophytes, and it is consequently 

 easy to understand how it comes that certain species, such as Rhipsalis 

 Cassytha and other Cactaceae, can live upon either trees or rocks. The 

 characteristic structural features of epiphytes will therefore be considered 

 in detail in Section III, which deals with xerophytic vegetation.' 



SAPROPHYTES 



In the case of many epiphytes it must be assumed that they derive 

 nutriment from dead parts of plants (bark) upon which they grow ; 

 they thus feed upon dead organic substance, and are saprophytes. 



Larger numbers and more pronounced forms of saprophytes are, 



' Schimper, 1884, 1888a; Mez, 1904 a. ' Gobel, 1891. 



* Gobel, 1889-Q3 and 1898-1901. ' Gobcl, 1889-93. 



' A. F. W. Schimper, 1884, 1888, 1898. * Sec G. Karsten, 1894. 



' See Wittrock, 1894; Willis and Burkill, 1904; Ule, 1904; Cockayne, 1901. 



