2 Charles E. Besscy 



posed of perennial-rooted grasses, sedges, and rushes. Where 

 these form a close sod new species are almost wholly excluded, 

 and but little change takes place in the character of the vegeta- 

 tion. It is only where the surface is not closely covered that the 

 grassy vegetation is more easily modified by the incoming of 

 new species. Where accident, or disease, or some other cause 

 has destroyed the grassy covering new species promptly take 

 possession. A fine example of this is fo be seen in the growth 

 of Hclianthits annnus on the mounds made on the prairies by 

 such burrowing animals as gophers and prairie dogs. Where 

 the tough sod was broken by the freight-wagons which crossed 

 Nebraska by various "trails" many years ago botanists find many 

 newcomers, which could not have gained a foothold in the 

 unbroken sod. 



FACTORS IN MIGRATION 



The means of migration are physical and biological, the for- 

 mer including all the factors which are external to the plant, and 

 which involve the mechanical movement of the plant or some of 

 its parts, while the latter includes the devices on the part of the 

 plant by means of which it takes advantage of physical agencies. 

 In this paper only those means which have to do with the mi- 

 gration of forest trees will be considered, thus limiting the dis- 

 cussion by excluding a multitude of devices of all degrees of 

 complexity which pertain to other plants. 



The most general physical agent in the dissemination of plants 

 is the movement of the air in the currents of wind. These sweep 

 over the earth's surface with all degrees of rapidity, and with a 

 carrying power which increases as the squares of the velocities. 

 The fact that winds shift their direction at short intervals, rarely 

 maintaining the same direction for more than a few hours, or at 

 most a few days, makes them especially useful agents in the 

 movement and transportation of such seeds and fruits as can be 

 blown from place to place. It is probably true that to some ex- 

 tent the distribution of all of the species of trees which grow 

 naturally in Nebraska is affected by the winds. The violent and 

 long continued winds of high velocity transport seeds and fruits 



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