274 THE FORMATION 



method for many woodland formations where stumps already abound or 

 where a fire has occurred, and it is but rarely necessary to cut down trees 

 for this purpose. When trees or shrubs are present as relicts, the same 

 method is used to determine the length of time taken by the development of 

 the corresponding stages, 



THE STRUCTURE OF THE FORMATION 



331. Since all the structures exhibited by formations, such as zones, lay- 

 ers, consocies, etc., are to be referred to zonation or alternation, these princi- 

 ples are first considered in detail. This, then, constitutes the basis for a con- 

 sideration of the structure of a normal formation, with special reference to 

 the different parts that compose it. The investigation of formational struc- 

 ture, since the latter is the result of aggregation, invasion, and succession, is 

 accomplished by instruments, quadrats, etc., in the manner already indicated 

 under development, and no further discussion of it is necessary here. 



ZONATION 



332. Concept. The recognition of vegetation zones dates from Toume- 

 fort\ who found that, while the plants of Armenia occupied the foot of 

 Mount Ararat, the vegetation of the slopes above contained many species of 

 southern Europe. Still higher appeared a flora similar to that of Sweden, 

 and on the summit grew arctic plants, such as those of Lapland. 



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 analyze zonation, or to formulate a definite method of investigating 

 it. Zonation is the practically universal response of plants to the quantita- 

 tive 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 greatest intensity. The result is that the plants of the 

 habitat arrange themselves in belts about this point, their position being de- 

 termined by their relation to the factor concerned. Close investigation will 

 show that there is hardly a formation 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 recognized in temperature zones. A conti- 



>Relation d'nn Voyage du Levant. 1717. 



