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 kiled 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. 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. 
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. 
