434 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



epiphytes, living on the topmost branches and twigs of the taller 

 trees, such as some Bromeliads and, remarkably enough, Cacti ; 

 (b) sun-epiphytes, usually xeromorphic and occurring chiefly in the 

 centres of the crowns and along the larger branches of the upper 

 tree-stories, and usually comprising the richest of the epiphytic 

 synusiae in both species and individuals ; and (r) shade-epiphytes, 

 mainly found on the trunks and branches of C-stratum trees, or on 

 the stems of the larger lianes. The shade-epiphyte synusia consists 

 chiefly of Ferns, and most of its members show no trace of xero- 

 morphy. The average vertical ranges of the difl^erent synusiae 

 depend on the light factor. Thus they tend to be constant within 

 any one type of forest but diflfer in diflferent types, being high in 

 forests where the top strata are dense, relatively low in more open 

 types of forest, and still lower on isolated trees or the margins of 

 clearings or rivers. 



Further adaptations (or anyway beneficial specializations) widely 

 exhibited by epiphytes are wind-borne spores (such as those of 

 Ferns) or seeds (such as those of Orchids) or fruits, though other 

 seeds and fruits are commonly dispersed by animals. Indeed it is 

 difficult to conceive of epiphytes being able to maintain themselves 

 without some effective means of dispersal of their propagules. 

 Some types, often termed hemi-epiphytes, develop long aerial roots 

 which reach the ground and so link epiphytes with the next group 

 of ' forest furnishings ', the stranglers. 



The epiphytic vegetation of tropical rain forests often includes 

 abundant Algae, Lichenes, Musci, and Hepaticae ; indeed, with the 

 usual absence of the mossy layer on the forest floor, all the Lichens 

 present and almost all of the Algae and Bryophytes are normally 

 epiphytic except occasionally in spots where fallen leaves have not 

 collected. There are, however, all manner of ' associules ' on stones, 

 fallen logs, and so forth, as well as on tree-trunks down to ground- 

 level. Otherwise, non-vascular plants contribute widely to the 

 classes of sun- and shade-epiphytes and also grow as ' epiphyllae ' 

 on (normally living) leaves — in the last instance mainly in the shady 

 undergrowth. Actually, the most abundant epiphytes of the shade 

 community tend to be Bryophytes, which often carpet the branches 

 of shrubs or hang down in the air, while the epiphytes of the sun 

 community include also many foliose Lichens, the Bryophytes of this 

 synusia tending to be more compact and xeromorphic. The 

 epiphyllae are mainly Algae, Lichens, or leafy Liverworts, and are 

 found chiefly on the upper surface of rather long-lived evergreen 



