vegetative growth of this strain being severely hmited when the 

 photoperiod is below 13.5 hr. Plants from central Oklahoma have 

 either intermediate or long-day requirements, with a lower crit- 

 ical photoperiod of less than 13 hr, vegetative growth of this strain 

 being vigorous in periods of 13 hr or more.^^^ By means of this 

 ecotypic differentiation in essential requirements, side-oats grama 

 is adapted to a wide latitudinal range. Little bluestem {Andro- 

 pogon scoparius) shows an even wider adaptation of requirements 

 to latitudinal and other environmental conditions. Thus in the 

 reseeding of depleted lands it is important to consider the require- 

 ments of the strains or species that are used. The seed should come 

 from areas as similar as possible, especially in latitude, temper- 

 ature, precipitation, and evaporating power of the air. Such simi- 

 lar areas are known as homoclimes or agroclimatic analogs.^ ^2 

 Knowledge of the requirements of cultivated plants is far ad- 

 vanced in contrast to that for wild plants (Figure 1-3). The former 

 are usually grown in single cultures under definite spacing so that 

 competition between plants for essential requirements is reduced 

 sufficiently to permit maximum yields per acre. Wild plants, how- 

 ever, grow in mixtures of many species in which it is difficult to 

 determine the requirements of individual ones. Data obtained by 

 growing wild plants in pure cultures are only partially applicable 

 to natural mixtures because of great differences in requirements, 

 as indicated by differences in growth rate and form, when they 

 are grown in association. Techniques for determining the require- 

 ments of species in natural stands need to be designed, so the 

 effects of competition and other interactions may be better 

 understood. 



Ecological Amplitude 



The characteristic potentiality for growth of a species within a 

 limited range of environmental conditions is known as the eco- 

 logical amplitude or tolerance range of the species. Ecological 

 amplitude seems preferable because it presents more clearly the 

 idea of the range of conditions in which an organism can live and 

 thrive, while tolerance refers more to the extremes within which 

 an organism can survive. The former is affirmative and definite, 



14 • Species axmd Popvmlatioxas 



