BOOK V. 



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 large number of seeds which certain plants produce 

 insures their incessant reproduction, and in this respect cal- 

 culation often gives very unexpected results. Ray counted 

 32,000 seeds on one poppy 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 produces more than 529,000. 



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

 erations 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 rigor of 

 some climates, and man, whose civilization encroaches upon 

 nature, place a barrier to it. The first invaders of a virgin 



