PART III. 



GEOGRAPHICAL BOTANY, OE THE DISTRIBUTION OF PLANTS 

 OVER THE GLOBE. 



1131. THIS department of Botany treats of the manner in which 

 plants are affected by climate and station, and endeavours to investi- 

 gate the conditions under which particular families, or species of plants, 

 are confined to certain zones of latitude and altitude. It is a subject 

 of great interest, and one which cannot be prosecuted with success until 

 the vegetation of the globe is more fully known. So long as there are 

 vast tracts of continents unexplored by botanical travellers, the facts 

 upon which Botanical Geography is founded must be imperfect. 



I. EPIRRHEOLOGY, OB THE INFLUENCE OF VARIOUS EXTERNAL. 



AGENTS ON PLANTS. 



1132. It is a matter of common observation, that the localities and 

 soils in which plants grow vary much. Thus, some species grow in 

 the shade, while others thrive best in full exposure to light ; some 

 grow in mountainous or alpine districts, while others prefer the plains ; 

 some are found in dry, others in marshy places ; some are submersed 

 in lakes or in the sea; while others live on muddy banks, or on 

 sandy shores. The plants growing on a granitic or micaceous soil differ 

 frequently from those found on trap, limestone, or sandstone. It is 

 equally well known that climate exercises a powerful influence on ve- 

 getation, modifying the Floras in different regions of the globe. Some 

 plants are fitted to bear the rigour and duration of an arctic winter, 

 with a moderate summer heat, others require the heat and light of the 

 torrid zone ; and between these two extremes, there are all varieties 

 of gradation. Thus vegetation extends over the whole globe, from one 



