OF SPECIES AND POPUI^ATIONS 



The ways in which plants carry on their hfe processes, developed 

 and genetically fixed during the course of evolution of each kind, 

 differ from species to species. The genetic constitution of each 

 species does permit a certain range of expression or function- 

 ing according to the species and the impinging environmental 

 conditions. Some plants, for example, can bloom and produce 

 seed in the shade, others require full sunlight. Some can carry on 

 photosynthesis and grow under a wide range of soil moisture con- 

 ditions, others require a continuously saturated soil. Legumes can 

 utilize the nitrogen fixed by bacteria in their root nodules, while 

 most species must secure their nitrogen from the soil. 



These ways of carrying on life processes are a fundamental part 

 of the characteristics of the species and determine in large meas- 

 ure the formation of groups of organisms or communities. The 

 physical and the biotic influences interact upon one another, so it 

 is often difficult to determine the most important relationships 

 that influence success. Furthermore, species differ genetically in 



