CHAPTER XVI 



THE TEMPERATURE RELATION 



A LARGE number of species of plants composing the 

 main vegetation of the earth are seldom, if ever, exposed 

 within their normal ranges to great extremes of tempera- 

 ture. There are many annuals which first appear after 

 the dangers of severe frosts are past, and they perfect 

 their fruits long before the growing season is closed. A 

 considerable number of perennials may be exposed to ex- 

 tremes only in a resting or semidormant condition. In 

 general, then, native plants have been long acted upcfo. 

 by the particular climatic factors of the region, so that 

 they show in a telling manner the influence of a long line 

 of ancestry whose development and survival within the 

 region is at least relatively fixed. 



f 238. Climatic extremes and introduced plants. - - In- 

 troduced plants in any region are, generally speaking, 

 much more likely to suffer exposure to an injurious ex- 

 treme, especially cold ; yet exceptional conditions may 

 bring disaster to any type of vegetation. The peach in 

 the South and Southwest is sometimes in blossom before 

 the winter is at an end, and the blossoms are not infre- 

 quently caught by late frosts. The famous peach belt of 

 Michigan was visited in 1905 by an early frost in October, 

 and the result was the practical annihilation of the peach 



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