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 difficulties 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 
