PRIMER OF PRIMERS. 183 



When an artesian well is successfully dug in the 

 Sahara Desert, the appearance of the water is 

 almost invariably followed by the appearance of a 

 flora that often contains species peculiar to such 

 districts. 



Wherever the virgin soil of the prairies is up- 

 turned, and thus exposed to the air, as by the 

 wheels of the settlers' wagons or other causes, new 

 species of plants appear. 



In the North Temperate Zone the burning of 

 pine forests is almost invariably followed by the 

 appearance of scrub-oak. 



This wide distribution of plant germs, together 

 with their wonderful vitality, insure the natural 

 growth of a vegetable covering in all regions of 

 the earth where suitable conditions of soil, light, 

 heat, and moisture exist. 



The character of the vegetation in any district 

 depends more on peculiarities in the distribution 

 of light, heat, and moisture in such districts than 

 on the character of the soil. 



The peculiarities in the distribution of the rain- 

 fall in any country determine to a great extent the 

 character of the flora of such country. 



When the rainfall in any region is entirely ab- 

 sent, no matter what the character of the soil may 



