The Migration of Our Forest Trees. 203 



but also in the canyons of Wyoming, and here we find that almost one- 

 half of our trees of this kind came from the west. It is to be remarked, 

 however, that while five of the six southeastern species have crossed 

 the state, the five western species have moved eastward only a few 

 miles from the Wyoming line. 



Of the thirteen southeastern species with winged seeds or fruits 

 three barely enter the state, one has advanced one-fourth of the way 

 across the state; three, one-half; one, two-thirds, and five to or beyond 

 the western border. Of the four western species, two have barely en- 

 tered; one has advanced half way, and one, two-thirds of the way across 

 the state. 



Of the sixteen species with fleshy fruits, seven have Darely entered 

 the state; six have advanced half way across the state; one, three- 

 fourths, and two to the western border and beyond. Of the four west- 

 ern species, two have advanced about one-fourth of the way across the 

 state; one, three-fourths, and one has reached the Missouri river. 



Lastly, if we examine the eighteen species with edible nuts, all of 

 which have entered from the southeast, we find that fifteen have barely 

 entered the s.tate; one has advanced nearly one-fourth of the way across 

 the state, one, two-thirds, and one, three-fourths. 



Summarizing what we have found, by assigning a definite value to 

 the distance covered by each species and taking the aggregate of these 

 for all the species, we find that the average of those with winged seeds 

 and fruit is 53 per cent of the whole distance; for those with hairy seeds, 

 52 per cent; with fleshy fruits, 45 per cent; with edible nuts, 16 per 

 cent; and with rolling balls, 10 per cent. We can thus express the 

 eflBciency of each device in these per cents, as follows: 



Wings on seeds or fruits 53 per cent 



Hairs on seeds 52 " 



Fleshy fruits 45 " 



Edible nuts 16 " 



Rolling balls 10 " 



That the migrating movement of the trees in Nebraska is still go- 

 ing on is attested by many observers, especially in the southeastern 

 part of the state. The conditions under which such movement occurs 

 are usually the following: (1) Cessation of prairie fires, (2) protec- 

 tion from domestic animals, (3) a forest border in a moist valley. 

 Under such conditions the forest border becomes margined with tall- 

 growing weeds which kill the tenacious prairie grasses, at the same 

 time affording a lodgment for seeds of shrubs and trees. These grow, 

 and gradually the shrubs and trees retain possession of the belt of 



