392 FOUNDATIONS OF BOTANY 



in rapidly from other regions, often produced actual fam- 

 ine. Wild 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. 



461. 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 whole 

 lives upon other plants. 



462. Adaptations to meet Adverse Conditions. — Since 

 there are so many kinds of diificulties to be met before the 

 seed can grow into a mature plant and produce seed in its 

 turn, and since the earth's surface offers such extreme 

 variations as regards heat, sunlight, rainfall, and quality 

 of soil, it is evident that there is a great opportunity 



