COMPETITION AIsD ECESIS 259 



occur regularly, and if the conditions which cause them con- 

 tinue, they may become fixed. Competition, like the ])hysical 

 factors through which it acts, may produce adaptations which 

 characterize new forms. 



Competition enters into practically all the changes of vegeta- 

 tion. It is absent only in the very beginning of open formations 

 on new or denuded soils. It is one of the fundamental factors 

 in invasion, and the reaction of competing occupant and invader 

 is the decisive element in all successions. Competition plays a 

 ]:)art in both alternation and zonation, but it is especially character- 

 istic of the former. It not only modifies the alternation arising 

 from physical factors, but it also exerts strong control over that 

 due to the historical factor, i.e., the time of invasion. 



274. The study of competition. The course of competition 

 under natural conditions may be followed in the field, or com- 

 petition may be artificially produced and studied in the plant- 

 house. There is no essential difference in the nature of the com- 

 petition in the two places, but the plant-house is ordinarily more 

 favorable because the study can be kept under control. Various 

 competition cultures are distinguished with respect to the point 

 of attack,! 13^^ Q^ly two of these are used for elementary work. 

 Simple cultures are those in which a single species is used. The 

 resulting group is a family, and the competition is between like 

 individuals. In such cultures the problem of the factors in com- 

 petition is reduced to its simplest terms. Mixed cultures are 

 composed of two or more species, and are correspondingly more 

 complex. Either culture is made permanent by allowing the 

 plants to ripen and drop their seeds from generation to gener- 

 ation, just as in nature. 



275. Competition cultures are a meter square, i.e., they are 

 indoor ciuadrats. For a simple culture, the plot is usually divided 

 into four squares to give different degrees of density. The density 

 may be varied as desired, though, as a rule, 12 seeds are planted 

 in one, 25 in another, 50 in the third, and 100 in the fourth. In 

 making a mixed culture, the problem is simplified by using two 

 species only. In one half of the plot the number of seeds of the 

 one is double that of the otlier; in the other half, conditions arc 

 reversed. At the time the plots are sown, seeds are started in 



Research Mctliods, 310. 



