BOOK Y, 



MIGRATIONS OF PLANTS. 



Nothing reveals to us the resources of nature in a more 

 imposing way than the facility with which she covers all 

 the surface of the globe with vegetation and life. At 

 times she seems to trust solely to the immense fecundity 

 allotted to the species; at others she employs the most 

 ingenious and varied proceedings in order to transport 

 her fruits and seeds from one pole to another. 



The considerable number of seeds which certain plants 

 produce insures their incessant reproduction^ and in this 

 respect calculation often gives very unexpected results. 

 Ray counted 32,000 grains on one pojDpy stalk, and 

 Linnaeus says that a single stem of tobacco sometimes 

 yields 40,000. Dodard carries these figures still higher 

 in respect to the number of fruits that can be collected 

 from an elm. According to him, this tree annually pro- 

 duces more than 529,000. 



It is clear that if all the seeds grew up, only a few 

 generations would pass away ere these forms of vegetable 

 life covered the entire surface of the globe. But a host 

 of causes arrest this menacing invasion. Animals, the 

 rigour of some climates, and man, whose civilization 



