OUTLINE OF PLANT GEOGRAPHY 



CHAPTER I 



INTRODUCTION 



As one studies the existing vegetation of the Earth, one I 

 nizes certain factors whose influence in determining the < list ribution 

 of the species composing this vegetation is sufficiently clear; but 

 there are other factors quite as potent which are by no means - i 



obvious. 



Of course for the existence of normal green plants, a certain 

 minimum of light, heat and moisture is essential; and up to a 

 certain point the luxuriance of vegetation is in direct ratio to the 

 relation of these three factors, and culminates where all work 

 together, as in the equatorial regions of both hemispheres. 



The soil constituents are also very evident factors in determining 

 the distribution of many species. Certain plants like rhododen- 

 drons and others of the heath family, are notoriously impatient 

 of lime, and only thrive in acid soils. Others like lettuce, aspara- 

 gus, clover, demand lime in the soil as a condition for their healthy 

 growth. Heavy clay soils suit some plants, light sandy soils are 

 indispensable for others. 



Conditions in widely separated regions may be very similar. 

 but this by no means implies a close relationship of the vegetation 

 between these regions, although its general aspect may be much 

 the same. Thus the rain forests of the Amazon Valley, and th 

 of Borneo or Java, look much alike-lofty trees draped with 

 lianas, orchids and other epiphytes, with an undergrowth of 

 palms, huge ferns, arums and other large leaved growths like the 

 bananas and gingers. In spite of this cl< ueral resemblam 



probably there would not be a single species common to the two 

 regions. So in the temperate parts of Europe and North America 

 there are relatively few common species, although the floras ha 

 much in common, and are obviously related to each other. 



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