8 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



ecological factors and, consequently, to a considerable extent on 

 climate {see next section). An understanding of them is fundamental 

 to our main subject, so some consideration of them seems desirable 

 at this stage. 



Just as the land-masses of the world, for example, make up a 

 definite (if seemingly unorganized) pattern on the surface of the 

 globe, so do other features, that are likewise definable in area, make 

 up their own special patterns. Such pattern-forming features 

 include the various factors of the environment, with which we shall 

 deal in Chapter X. Thus, certain ranges of temperature, for 

 example, obtain only within certain areas, and the same is true 

 especially of other climatic features {see next section). 



In the simplest case it might be supposed that a particular kind 

 of land-plant, needing land to live on, could occupy all of the water- 

 and ice-free land of the globe. But in actual fact quite numerous, 

 often interdependent and overlapping, environmental and other 

 factors prevent this, and no known kind of plant, however wide its 

 habitat tolerance, occupies more than a very small proportion of the 

 world's surface. At the other extreme are the numerous species 

 which appear to inhabit only one limited tract of the globe or even 

 a single spot. Each and every species has its particular area, its 

 geographical distribution, whether this be small or large, and 

 whether continuous or broken up into a more complicated pattern. 

 And the pattern will be related to some particular factor or factors 

 of the environment, to some migrational ability the plant may 

 possess, and/or to evolutionary and geological or more recent 

 history. 



These migrational tendencies, together with the historical aspects 

 of distribution, will be discussed in Chapters IV, V, and VI, and 

 it will be found that such aspects may greatly aflfect the areas at 

 present occupied by particular plants. But the factors of the 

 environment are apt immediately to limit and circumscribe the area 

 that can be occupied by a plant and, although treated in fair detail 

 in a later chapter, require some explanation here before we can 

 proceed. 



Any condition of the habitat, whether climatic, physiographic, 

 edaphic (concerned with the soil), or biotic (concerned with living 

 organisms), may limit the area occupied by a plant, and usually 

 many of these conditions do come into play. A simple instance is 

 that a tropical plant requires warm conditions — or at least, it cannot 

 grow in the cold. Usually, however, matters are far more com- 



