PRESENT PROBLEMS IX PLANT ECOLOGY 



III. Vegetation and Altitude 

 PROFESSOR CHARLES H. SHAW 

 The Medico-Chirurgical College, Philadelphia 



In the study of the relation of plants to environment, 

 there are few problems of greater interest than those 

 presented by the vegetation of mountains. The general 

 facts are somewhat familiar, and reference to them is 

 here necessarily brief. Whatever the vegetation of the 

 surrounding country, mountains are usually forested; 

 the forest is often composed of several zones in which 

 different kinds of trees successively predominate ; higher 

 up the forests finally cease and give place to grassland 

 of perennial herbs and low shrubs— these are some of the 

 more general facts of mountain vegetation. There is 

 bound up with them not only strangeness and beauty, 

 but also a series of most interesting problems in the 

 ecology of plants. 



If I understand rightly the reason for this symposium, 

 my duty is to state, as well as in brief compass I may, 

 the present state of our knowledge in regard to these 

 phenomena. In general, they rest back upon physical 

 environment. In so saying, however, it must be kept in 

 mind that biotic factors early modify the primitive phys- 

 ical ones ; the reason for the occurrence or absence of a 

 species may be the conditions created by other species. 



The important factors which vary with altitude seem to 

 be heat, light, precipitation, evaporation, and a factor 

 made up of several of those named, namely, length of sea- 

 son. Let us consider these severally, and try how far 



t tlf bIt ° f PapCrS presen * ec \ be f° r e the Botanical Society of America, 

 420 



