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, like 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. Adajitations 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 like 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 the 

 opposition which plants encounter from overcrowding or from 



