CHAPTER VII. 

 GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



Principal factors influencing the distribution of plants. British 

 Flora, past and present. Distribution of Cryptogams. Distribu- 

 tion of Phanerogams. 



r I ""HE influence of temperature on plant life has 

 already been alluded to, and this factor is amongst 

 the most important in determining the distribution of 

 plants at the present day. The fact of certain groups 

 of plants occurring only in particular regions does not 

 prove that that particular region is the only one suited 

 to their requirements, as it has been proved in many 

 instances that when such locally occurring plants have 

 been introduced by human agency into new localities, 

 they have not only established themselves, but in several 

 instances have monopolized the new area, and more or 

 less completely driven out the previously-existing indi- 

 genous flora. Numerous common European weeds that 

 have been unintentionally introduced into new areas have 

 spread at an enormous rate, driving the native plants 

 before them, and causing serious inconvenience to the 

 settlers in such districts ; as an illustration may be men- 

 tioned the spread of the common thistle on the pampas 

 of Buenos Ayres, where at one time it almost completely 



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