THE STRUGGLE FOR EXISTENCE 203 



and varied that a special treatise might well be devoted to 

 this subject alone. 



249. Destruction of Plants by Unfavorable Climates. — 

 Land-plants throughout the greater part of the earth's 

 surface are killed in enormous numbers by excessive heat 

 and drought, by floods, or by frost. After a very dry 

 spring or summer the scantiness of the crops, before the 

 era of railroads which nowadays enable food to be brought 

 in rapidly from other regions, often produced actual fam- 

 ine. AVild plants are not observed so carefully as culti- 

 vated ones are, but almost every one has noticed the 

 patches of grass, apparently dead, in pastures and the 

 withered herbaceous plants everywhere through the fields 

 and woods after a long drought. 



Floods destroy the plants over large areas by drowning 

 them, by sweeping them bodily away, or by covering them 

 with sand and gravel. Frosts kill many annual plants 

 before they have ripened their seeds, and severe and 

 changeable winters sometimes kill perennial plants. 



250. Destruction by Other Plants. ■ — Overcrowding is 

 one of the commonest ways in which plants get rid of 

 their weaker neighbors. If the market-gardener sows his 

 lettuce or his beets too thickly, few perfect plants will be 

 produced, and the same kind of effect is brought about in 

 nature on an immense scale. Sometimes plants are over- 

 shadowed and stunted or killed by the growth all about 

 them of others of the same kind ; sometimes it is plants 

 of other kinds that crowd less hardy ones out of existence. 



Whole tribes of parasitic plants, some comparatively 

 large, like the dodder and the . mistletoe, others micro- 

 scopic, like blights and mildews, prey during their entire 

 lives upon other plants. 



