4 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the end of this chapter that authorities have differed greatly in the 

 past as to the bounds and, in their view, legitimate content of the 

 science of plant geography. In the present introductory work it 

 will be interpreted in a much wider sense than usual, as including 

 not only all geographical manifestations of plants whether single or 

 collective (and hence a good deal of economic and some morpho- 

 logical botany), but also the reasons behind these manifestations. 

 This will presuppose some consideration of the bases of distributions 

 in space and time, and consequently of the relationship to environ- 

 ment (ecology), of the classification and systematic arrangement of 

 different kinds of plants particularly through their external form 

 (taxonomy and systematy), of the study of their internal workings 

 (physiology), of their economic importance, and of other disciplines 

 that are not normally thought of as plant geographical — hence in 

 part the reference to ' some related sciences ' in the sub-title of 

 this book, to certain of which in some modest degree it may also 

 serve as a general introduction. 



The ultimate purpose of geography is the study of the differences 

 in the areas which make up the world. Yet when the plant popula- 

 tions are taken into consideration it comes as no surprise, in view 

 of their extreme variability, to find that one of the main results of 

 such a study is the realization that each area is unique. Once we 

 leave the * systematic ' study of particular phenomena, such as the 

 relationship of individual kinds of plants to different areas, and 

 enter the ' regional ' sphere of correlation of the various manifesta- 

 tions which point to these differences in area, the problem of 

 organizing our study becomes almost overwhelming. With any set 

 of phenomena as infinitely variable as vegetation (in both time and 

 space, as we shall see), the areal integration desired in their geography 

 is rendered practicable only by ignoring variations within the smaller 

 unit-areas, which may then be studied together and ' lumped ' into 

 larger ones. 



Plant geography attempts to integrate these floristic and vegeta- 

 tional features as far as possible on a world basis, and for recording 

 and illustration makes use of maps as one of its main tools. But the 

 very construction of these maps presupposes the utmost care in the 

 comparison of the entities whose ranges they indicate. Lack of 

 such care is one of the greatest limitations with which the plant 

 geographer is faced. Another is the still fragmentary state of Man's 

 knowledge of the distribution of the vast majority of the many 

 hundreds of thousands of different kinds of plants inhabiting the 



