THE MANNER OF INVASION 85 



succeeding year or season, until the area has again become 

 stabilised. Denuded quadrats are of great value in deter- 

 mining the comparative mobility and ecesis of the species of 

 a formation or patch, and they are indispensable for the re- 

 construction of the early stages of a succession. 



A device known as the migration circle is employed to 

 study the amount, direction, and rapidity of movement away 

 from any particular group of individuals, which is essenti- 

 ally a miniature vegetation centre. The circle is used in 

 place of the quadrat, because it is probable that migration 

 will operate in all directions. The migration circle, like the 

 quadrat, may be either temporary or permanent : the latter, 

 which gives by far the more valuable results, is secured by 

 driving a properly labelled stake in the midst of the par- 

 ticular plant group, and recording the radius of the area. 

 The size of the circle varies with the height and mobility of 

 the species studied ; the most satisfactory circles are those 

 with one, five or ten-meter radius. The twenty-five meter 

 circle is useful at the margin of denuded areas and in very 

 open vegetation, but the area enclosed is so large as to make 

 it impracticable in closed formations. The choice of a circle 

 depends upon the size of the nucleus of individuals taken as 

 a center and upon the distance between two or more 

 adjacent centers. Because of the large area to be cleared, 

 it has not been found feasible to use denuded circles with a 

 radius larger than one meter. The value of a denuded circle 

 ten meters in radius for the comparative investigation of 

 migration and ecesis would be so great, however, that its 

 use is certain, as experimental methods in the field come to 

 be recognized as more and more imperative. 



