Plant Migration Studies 19 



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 spe- 

 cies, two have barely entered; one has advanced half way, and 

 one, two-thirds of the way across the state. 



Of the sixteen species with fleshy fruits, seven have barely 

 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 western 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 state ; 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 fruits is fifty-three per cent of the 

 whole distance ; for those with hairy seeds, fifty-two per cent ; 

 with fleshy fruits, forty-five per cent; with edible nuts, sixteen 

 per cent ; and with rolling ,balls, ten per cent. We can thus 

 express the efficiency of each device in these per cents, as fol- 

 lows : 



Wings on seeds or fruits 53 per cent 



Hairs on seeds 5^ 



Fleshy fruits 45 



Edible nuts 16 



Rolling balls 10 " 



That the migrating movement of the trees in Nebraska is still 

 going on is attested by many observers, especially in the south- 

 eastern part of the state. The conditions under which such 

 movement occurs are usually the following: — (i) Cessation of 

 prairie fires, (2) protection from domestic animals, (3) a forest 

 border in a moist valley. Under such conditions the forest bor- 



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