l] WHAT IS PLANT GEOGRAPHY? g 



plicated than this, in that such a plant commonly requires also 

 environmental conditions within a certain range of moisture, light, 

 soil type, etc. As the world affords, usually over considerable areas, 

 practically every conceivable combination of habitat factors, the 

 area occupied by a particular plant will, inter alia, depend upon its 

 physiological make-up and reaction to the component factors. 

 Herein lies the close connection between particular plants and their 

 favoured habitats. 



Whereas probably no two situations or even areas of a seemingly 

 uniform habitat are exactly identical, we must in practice accept 

 them as being alike, even as they appear to be so accepted by plants. 

 Accordingly the similar habitats of the world may be grouped together 

 to form recognizable patterns ; and, looking at things the other way, 

 we find that there usually is a pattern of areas occupied by each 

 plant. Often a single habitat factor lies behind such a pattern of 

 plant distribution and may readily be recognized as doing so. But, 

 unfortunately for those who crave simple monistic explanations, the 

 area potentially inhabitable by a particular kind of plant, as pre- 

 scribed by suitable habitat conditions, and that area which it actually 

 occupies in the world today, are rarely if ever identical or even 

 similar. Yet although the area which might be occupied is of both 

 interest and importance to the scientist and to mankind, the plant 

 geographer's immediate concern is with the fact, i.e. the actual area 

 occupied. 



In the same way, the many different types of vegetation form 

 geographical patterns of their own, but here again the plant geo- 

 grapher is concerned more with the effects, i.e. the actual patterns, 

 than with the causes which properly belong to the historical side of 

 his studies. The vegetation pattern is to a considerable extent the 

 sum of the overlapping distributions of the component plants ; but 

 it commonly has a form of its own, for in biology the sum total 

 of the components does not necessarily constitute the expected 

 whole. The organisms' interrelationships and reactions add much 

 that is new to the system and form an integral part of the end result. 



Climate the Master 



As climate tends to supply the most important over-all factors 

 determining plant distribution, it behoves us to give at this stage 

 some outline of the main ' world ' types of climate and related 

 vegetational features. Further details on climatic factors will be 



