II. — Plant Migration Studies 



BY CHARLES E. BESSEY 



I. FOREST TREES. 



It is a familiar fact that new species appear from time to time 

 among the native plants of a region. Such newcomers turn out 

 on examination to be new only in the sense that they have not 

 previously lived in the region, and in every instance these new 

 plants are found to have come from other regions where they 

 liad existed for a longer or shorter period of time. In some 

 cases the new species remain for a time and then disappear, or 

 at least become inconspicuous, but more commonly they crowd 

 in among the former plants and become permanent members of 

 the plant community. Whenever such an addition is made to 

 the flora of a region there is a readjustment of the former spe- 

 cies, with a necessary change in the relative numbers of the in- 

 dividuals, and the particular habitat of each. In the case of 

 annual plants these adjustmenrs are made rapidly, so that in a 

 short time the prominent features of the plant community may 

 be entirely changed. On the other hand, in the case of perennial 

 plants there is greater stability, new species finding greater dif- 

 ficulty in entering, and the old species giving away, if at all, 

 only after the lapse of a much longer time. A vegetation which 

 is well rooted in the ground is much less easily disturbed than 

 one whose roots live for but a single season and then abandon 

 the particular plot of ground where they grow. Forests are 

 therefore conservative plant communities, into which new species 

 gain entrance with difficulty, and which change very slowly after 

 such entrance has been effected. There is only one other plant 

 community whose stability approaches that of the forest, namely, 

 the grassy vegetation of the prairies and plains, which is com- 



University Stldies. Vol. V, No. 1, January 190.") 



I I 



