METHOD OF CONTROL HABITATS 



311 



the species are of very different height. Thus, rosettes have been grown 

 with stemmed plants, tall slender forms with low branching ones, erect plants 

 with twining and climbing plants, etc. Further evidence as to the nature 

 of competition has been sought by means of ccad cultures, and factor cul- 

 tures. In the former, plants of different response to water and light are 

 grown together under the same conditions, in order to evaluate the part 

 played by the nature of the plant. In a factor culture, the area is divided 

 into two or more parts which are given different amounts of water or of light, 

 in order to determine the influence of slight variations upon the same com- 

 petitors. In somewhat similar fashion, an attempt has been made to 



Fig. 84. Mixed culture of Solidago rigida and Onagra biennis. 



ascertain the bearing of biotic factors upon competition. Cultures are 

 easily made in which Cuscv.ta or parasitic fungi are used to place certain 

 species at a disadvantage. Permanent cultures are obtained by allowing the 

 plants to ripen and drop their seeds for several generations, just as in 

 nature. They are indispensable for determining the final outcome of the 

 competition between different species. 



362. Details of culture methods. All competition cultures have been 

 made i meter square. In other words, they are quadrats, and they are 

 treated exactly as denuded quadrats in the field with respect to factor 

 readings, charts, and photographs. In the writer's studies, germination 



