PAET III. 



OEOGEAPHICAL BOTANY, OR THE DISTRIBUTION OF 

 PLANTS OVER THE GLOBE. 



In tUs department of Botany we treat of the manner in which plants 

 are affected by climate and station, and the mode in which they are 

 distributed over the globe, and we endeavour to investigate the 

 conditions under which particular families, or species of plants, are con- 

 fined 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 trapts of continents unexplored by botanical travellers, the facts 

 upon which Botanical Geography is founded must be imperfect. 



I. — Epierheology, oe the Influence of Vaeious External 

 Agents on Plants. 



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 

 vegetation, 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 pole to the other — from the summit of the highest 

 mountains to the bosom of the ocean. Notwithstanding this general 

 diffusion of plant-life, there are a few spots in which it has not been 

 detected, such as the' hot sands of Africa, and some of the Antarctic 

 Islands. Each zone may be said to have its own peculiar vegetable 



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