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 
variations 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 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 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 algz (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. 
