THE STRUGGLE FOR 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, hke 
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 
