The Migration of Our Forest Trees. 18& 



"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 former 

 including all the factors which are external to the plant, and which in- 

 volve 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 migration of forest trees will be con- 

 sidered, thus limiting the discussion 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 extent 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 of all kinds, whether or not they show any special adapta- 

 tions for wind carriage. 



A second general agent in dissemination is moving water, upon 

 which seeds and fruits may float from place to place. Every brook, 

 every creek, every river carries thousands of seeds, many of which 

 ultimately float to the banks, or are lodged upon sandbars and islands. 

 In the aggregate the number of seeds carried in this manner is large, 

 but the number of trees resulting from their growth is by no means 

 as great, since many seeds are injured by prolonged soaking in water, 

 and in addition many do not find favorable conditions for growth w'hen 

 cast ashore. 



Animals of various kinds are active agents In the dissemination of 

 seeds and fruits, especially of trees. Here the means of transportation 

 are much more efficient, since they may result in the dispersal of seeds 

 in all directions, and often for much greater distances. Squirrels, rats, 

 mice and other rodents which carry and secrete stores of food, and 

 many herbivorous mammals which feed more or less commonly upon 



