438 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



of the taller trees, whereas in open types or areas they may descend 

 almost to ground-level. Their vertical distribution thus corresponds 

 with that of the autotrophic sun-epiphytes, and seems to be deter- 

 mined chiefly by intolerance of shade. 



In subtropical regions where the rainfall is abundant and well- 

 distributed, rain forests occur which are similar to the tropical ones 

 except for their tendency to be less luxuriant and dominated by 

 fewer species, and to include temperate elements but a smaller total 



Fig. 151. 



A tropical heniiparasitic Mistletoe, Visciim orieutale, the root of which 

 forms a single haustoriuni (absorbing organ). 



flora. Often there are fewer lianes and epiphytes as well as, especi- 

 ally, trees ; and often the middle (B) tree storey predominates, 

 although traces of both the others may be developed. Examples 

 are found bordering on the tropical rain forests and elsewhere as 

 already indicated above, and include the more luxuriant ' hammocks ' 

 of southern Florida. Such special features as plank-buttressing and 

 cauliflory, characteristic of the tropical rain forest, become less 

 evident or disappear. As temperate regions are approached, these 

 subtropical rain forests pass into the poorer but still evergreen, 

 warm-temperate ones described in Chapter XII. It should also be 

 noted that, in periodically drier areas inhabited by other types of 



