4^8 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



tend to have smaller leaves than those of the tropical rain forest. 

 The general foliage, too, is less dense, and commonly only two tree 

 strata are discernible, allowing light to penetrate and plentiful 

 ground- vegetation to develop (Fig. i6i). A fair amount of humus 

 may accumulate but stranglers are absent. 



Fig. i6i. — Two-storied montane rain forest at an altitude of 740 metres in the 



Philippine Islands. 



Although more and more temperate species enter as we ascend 

 in the upper forested zones on tropical mountains, the total flora 

 tends to decrease. But whereas this decrease is particularly marked 

 in the case of trees, it is not accompanied, as it is in unfavourable 

 lowland situations, by any marked tendency to dominance by single 

 species. Leaves in general become fewer in number and usually 

 more slender than at the lower levels, and the epiphytes are usually 

 smaller, almost all herbaceous, and mostly limited to Ferns and 

 Bryophytes or more lowly cryptogams. Small climbers as well as 

 epiphytes are, however, often abundant in the two-storied upper 

 montane forest, as shown in F"ig. 162. Epiphytic ' mosses ' 

 (mostly leafy Liverworts) tend to be particularly numerous and 

 luxuriant where mists prevail in these and any still higher forests, 



