3IO 



THE FORMATION 



METHOD OF CONTROL TIAI51TATS 



361. Competition cultures. Although it is quite possible to carry on 

 experiments in invasion and succession in the planthouse, the limited space 

 usually available makes this undesirable, except in a few problems where 

 control is necessary. Competition cultures, on the other hand, yield better 

 results in the planthouse than in the field, since the physical factors and the 

 appearance of unwelcome migrants are much more easily controlled. The 

 possibilities of the culture method in the study of competition seem inex- 

 haustible, and the author has found it necessarv to confine his own investi- 



ng. 83. Simple culture of floating ecads of Ranunculus sceleratus. 



gations to a few of the fundamental problems. In this work, he has 

 distinguished several kinds of cultures, based chiefly upon the species con- 

 cerned and the arrangement of the individuals. Simple cultures are those 

 in which a single species is used. The resulting group is a family, and the 

 competiticn is between like individuals. In such cultures, the problem of 

 the factors in competition is reduced to its simplest terms. Mixed cultures 

 arc based upon two or more species, and the problem is correspondingly 

 complicated. 'As a rule, all the seeds have been sown at the same time in 

 both simple and mixed cultures, but it has been found desirable to make 

 some heterochrotwus cultures, in which seeds are also sown after the plants 

 have appeared. Mixed cultures are distinguished as layered cultures, when 



