THE STRUGGLE FOE EXISTENCE. 203 



243. Destruction of Plants by Unfavorable Climates. — 

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

 face, 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 rail- 

 roads, which nowadays enable food to be brought in rapidly 

 from other regions, often produced actual famine. Wild 

 plants are not observed so carefully as cultivated ones are, but 

 almost every one has noticed the patches of grass, apparently 

 dead, in pastures and the withered herbaceous plants every- 

 where 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. 



244. 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 overshadowed 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 microscopic, like 

 blights and mildews, prey during their whole lives upon 

 other plants. 



245. Destruction by Animals. ■ — All animals are supported 

 directly or indirectly by plants. In some cases the animal 

 secures its food without seriously injuring the plant on which 

 it feeds. Browsing on the lower branches of a tree may do it 



