204 ELEMENTS OF BOTANY. 
little injury, and grazing animals, if not very numerous, may 
not seriously harm the pasture on which they feed. Fruit- 
eating animals may even be of much service by dispersing 
seeds (§ 224). But seed-eating birds and quadrupeds, ani- 
mals which, like the hog, dig up fleshy roots, root-stalks, 
tubers or bulbs, and eat them, or animals which, hke the 
sheep, graze so closely as to expose the roots of grasses to be 
parched by the sun, destroy immense numbers of plants. 
So too with wood-boring and leaf-eating insects, and snails, 
which consume great quantities of leaves. 
246. 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 offered for competi 
tion among plants. Of several plants of the same kind, grow- 
ing side by side, where there is room for but one full-grown 
one, all may be stunted, or one may develop more rapidly 
than the others, starve them out and shade them to death. 
Of two plants of different kinds the hardier will crowd 
out the less hardy, as ragweed, pigweed, and purslane do with 
ordinary garden crops. Weeds like these are rapid growers, 
stand drought or shade well, will bear to be trampled on, and, 
in general, show remarkable toughness of organization. 
Plants which can live under conditions which would be 
fatal to most others will find much less competition than the 
rank and file of plants are forced to encounter. Lichens, 
growing on barren rocks, are thus situated, and so are the 
fresh-water plants, somewhat lke pondscum in their struc- 
ture, which are found growing in hot springs at temperatures 
of 140°, or in some cases up to 200°. 
247. Examples of Rapid Increase. — Nothing but fe 
opposition which plants encounter from overcrowding or from 
