panied by low humidity causes more damage if the duration 

 covers several days rather than a few hours. Light possesses three 

 phases. The intensity of light varies from 8000 to 10,000 foot 

 candles at noon on a clear day in the open to as low as 100 foot 

 candles in a dense forest. The duration of light influences the 

 blooming of some kinds of plants; thus some bloom when the days 

 are short, others when the days are long. The quality of light 

 affects photosynthesis, the red rays being most effective in this 

 process. 



Plants respond to a complex of environmental factors impinging 

 upon them simultaneously, and it is often difficult to segregate 

 one particular factor as causing a certain response. Transpira- 

 tion, for example, is influenced by heat, humidity, soil moisture, 

 light, wind, and other factors all operating at the same time. On 

 a south-facing slope the rate of transpiration is usually greater 

 than on a north-facing one because of the greater intensity of 

 several of these factors. At the same time that the rate of trans- 

 piration is increasing as the heat and wind become more intense, 

 other processes such as photosynthesis, absorption, translocation, 

 assimilation, and growth are also being aff'ected. Every habitat 

 has a different combination of environmental factors — the com- 

 plex in a forest, for example, differing considerably from that in 

 a grassland. While both the environment and the community are 

 complex, it needs to be emphasized that the community repre- 

 sents an especially high degree of complexity because it is made 

 up of individual organisms, each functioning in its own particular 

 way in relation to an environment in which one factor may be 

 critical at one time, and another at some other time. Thus in a 

 certain spot one plant with a shallow root system may be wilting 

 while a deep-rooted plant in the same spot is flourishing, but in 

 early spring, when the surface soil is moist, the former may be 

 growing rapidly, the latter slowly. 



Essential Requirements for Every Species 



The basic needs for survival, such as water, mineral nutrients, 

 light, and heat must be available, or the plant cannot live and 

 grow. For example, the U-3 strain of Bermudagrass {Cynodon 



lO • Species andl PopulatiozftS 



