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PLANT PHYSIOLOGY AND ECOLOGY 



these agents usually act in a definite direction over great distances. 

 On the contrary, local migration is indeterminate as a rule, except 

 in the case of streams, glaciers, and slopes. The direction of 



Fig. 100. Determinate migration of the blue columbine, 

 Aquilegia coerulea, down a gravel slide. 



migration is thus seen to be controlled by the distributive agent. 

 The distance is determined by the intensity and duration of the 

 agent, as well as by the nature of the area through which it acts. 



Experiment 65. Amount and direction of migration. IMake a general 

 study of a railroad track for the migration of introduced plants, espe- 

 cially weeds, and of a stream which flows through a prairie or meadow 

 for the movement of forest species. 



Establish two migration circles. Select a plant or group with 

 comate or parachute fruits as the base for one, and an immobile species 

 for the other. If the plant chosen is just beginning to loosen its fruits, 

 the use of a circle of 2 m. radius shows the direction of movement 

 and the varying distance of the first flight. The bases of the two circles 

 are made permanent by driving a stake. In late spring the circles 

 are again examined, the new plants counted, and their relative position 

 with respect to the parent plant recorded. 



