204 ESSENTIALS OF BOTANY 



251. 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 

 Tariations as regards heat, sunlight, rainfall, and quality 

 of soil, it is evident that there is a great opportunity 

 offered for competition among plants. Of several plants 

 of the same kind, growing 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 wiU 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 that 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 

 numerous species of blue-green algae (Sect. 277) which are 

 found growing in hot springs, as in those of the Yellow- 

 stone National Park, at temperatures of 140° or more. 



252. Examples of Rapid Increase. — Nothing but the 

 opposition which plants encounter from overcrowding or 

 from the attacks of their enemies prevents any hardy kind 

 of plant from covering all suitable portions of a whole 

 continent, to the exclusion of most other vegetable life. 

 New Zealand and the pampas of La Plata and Paraguay, 

 in South America, have, during the present century, fur- 

 nished wonderful examples of the spread of European 

 species of plants over hundreds of thousands of square 



