IBO ZONATION 



pointed out that bilateral symmetry in the habitat was an 

 important cause of zonation, and insisted upon the funda- 

 mental nature of zonation, as a universal feature of vegeta- 

 tion. Clements (I902:bI9) distinguished three types of 

 topographical symmetry, viz., radial, bilateral, and unilat- 

 eral, to which correspond radial, bilateral and unilateral 

 zonation. 



As the historical summary shows, the concept of zonation 

 is the oldest in phytogeography. Notwithstanding this, it 

 has never been clearly defined, nor has there been any 

 detailed investigation of the phenomenon itself, or of the 

 causes which produce it. Zones are so common, and often 

 so clearly marked, that they invite study, but no serious 

 attempt has heretofore been made to analyse zonation, or to 

 formulate a definite method of investigating it. Zonation 

 is the practically universal ressponse of plants to the quanti- 

 tative distribution of physical factors in nature. In almost 

 all habitats, one or more of the physical factors present 

 decreases gradually in passing away from the point of great- 

 est intensity. The result is that the plants of the habitat 

 arrange themselves in belts about this point, their position 

 being determined by their relation to the factor concerned. 

 Close investigation will show that there is hardly a forma- 

 tion that is entirely without zonation, though in many cases 

 the zones are incomplete or obscure for various reasons. 

 Zonation is as characteristic of vegetation as a whole as it 

 is of its unit, the formation, a fact long ago recognised in 

 temperature zones. A continental climate, however, often 

 results in the interruption of these, with the consequence 

 that these belts of vegetation are not always continuous. 



CAUSES OF ZONATION 



The causes that produce zones are either biological, or 

 physical: the first have to do with some characteristic of 

 the plant, the second with the physical features of the 

 habitat. Biological causes arise from the method of growth, 

 from the manner of dissemination, or from the reaction of 

 the species upon the habitat. The formation of circles as a 



