432 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



species of small stature but having high light-requirements. A few 

 of the larger types and many of the small ones grow in the rain- 

 forest undergrowth, being supposedly species that are intolerant of 

 root-competition or smothering by fallen leaves. All have to put 

 up with lack of soil and hence paucity of mineral nutrients, and a 

 more or less precarious water-supply, though in this last connection 

 we should recall the persistently heavy and often daily rainfall in 

 most of their habitats. 



Epiphytes do not ordinarily have any ill-effect upon the supporting 

 ' host ', and, though constituting a very characteristic element in 

 the structure of the forest, play only a minor role in its economy. 

 This may even be the case when the epiphytes are so abundant as 

 to form an almost continuous investment of tree-trunks, as they 

 commonly do in uplands where the tree-canopy is thin or relatively 

 simple {cf. Fig. 162). They do, however, play an important part 

 in the ecosystem as habitats for animals, and they are further inter- 

 esting in showing many remarkable structural adaptations. Their 

 number and diversity are great, usually involving a wealth of crypto- 

 gams of all lower groups as well as Pteridophytes and flowering 

 plants, including some shrubs. Indeed it is the presence of a wide 

 range of epiphytes which especially distinguishes the tropical rain 

 forest from temperate forest communities, though epiphytes are 

 characteristic also of montane and subtropical rain forests, and may 

 be even more luxuriantly developed therein. Moreover, different 

 species of trees frequently show distinctions in their epiphytic 

 floras, supposedly because of different chemical constituents of rain- 

 wash as well as for the more obvious reasons of shade or bark- 

 texture, etc. 



A few of the more striking types of adaptations of epiphytes should 

 be mentioned. Many are constructed so as to collect a substitute 

 soil, which is mainly derived from the dead remains of other plants, 

 being often assembled by Ants which inhabit the plant's own root 

 system that grows into the so-constituted ' vegetable flower-pot ' of 

 these ' nest-epiphytes ' (Fig. 146). Others have to be able to 

 absorb water rapidly and for this purpose often have spongy ' vela- 

 men ' roots ; they also have to be able to conserve the water they 

 get, and consequently are often markedly xeromorphic or possessed 

 of special reservoirs {e.g. Fig. 147) or storage tissues (in ' tank- 

 epiphytes '). 



Three main classes of rain-forest epiphytes may be recognized, 

 corresponding to different microhabitats : {a) extreme xerophilous 



