6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the earlier chapters. This consideration of ' natural ' distributions 

 will be followed by a chapter on man-made ones — both intentional 

 (of crops) and unintentional (of weeds, etc.). And as the crops, or 

 potential crops, of the various parts of the world introduce some 

 of the greatest problems of mankind today, the next chapter will 

 emphasize the economic, and basic, significance of plant life. For 

 the geographical ranges of plants important to Man are often largely 

 determined by him, on whom their very existence may depend. 



In these initial chapters we will be dealing chiefly with special 

 kinds or systematic groups of plants and their distributions. This 

 is little more than a prelude to consideration of the natural group- 

 ings, the complex and variable assemblages of diff"erent plant com- 

 munities each composed of more or less numerous and diverse kinds 

 of plants, that make up collectively what we term vegetation. Before 

 actually beginning our study of vegetation we must consider the 

 environmental conditions (the ecological factors) which largely 

 control its distribution and form : such consideration will occupy 

 Chapter X. Some attention will also be given to physiological 

 make-up, which primarily determines the reaction of a plant to its 

 environment. 



The ecological factors at a point collectively constitute the habitat, 

 or ' habitat conditions '. The habitat, the place where an organism, 

 or commonly many organisms, live, may vary greatly from place 

 to place but tends to recur in at least comparable form in many 

 different places. Particular habitats are often relatively uniform 

 over considerable areas, as in the cases of salt-marshes, shallow 

 ponds, and sandy plains. Moreover, when a bare or disturbed area 

 is left alone, the vegetation inhabiting it tends to change, exhibiting 

 a series of vegetational types ranging from the first lowly colonists 

 to a relatively stable community which is ultimately the highest 

 the area can support, the advancing series being called a ' succession ' 

 or ' sere ', and its outcome the ' climax '. Chapter XI will deal 

 in a general way with the main types of plant habitats, successions, 

 and climaxes to be distinguished. 



The next five chapters will outline and illustrate the chief vegeta- 

 tional types of the world, starting with those to be observed in 

 temperate and adjacent lands as being most familiar and compre- 

 hensible to the greatest number of us. Following an account of 

 the vegetational types of polar lands and high altitudes elsewhere, 

 will be a chapter on tropical and adjacent lands, and thereafter one 

 on the plant communities of fresh and inland saline waters, wherever 



