310 THE FORMATION 
METHOD OF CONTROL TFABITATS 
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 necessary to confine his own investi- 
Fig. 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 
competition is between like individuals. In such cultures, the problem of 
the factors in competition is reduced to its simplest terms. Mixed cultures 
are 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 heterochronous cultures, in which seeds are also sown after the plants 
have appeared. Mixed cultures are distinguished as Jayered cultures, when 
