42 



THE AMERICAN BOTANIST 



willow or Cottonwood on the borders of the marshes or slug- 

 gish streams. Near the southern margin of the plain, how^ever, 

 there is a single long, low area of woody vegetation that stands 

 out so couspicuously from the surrounding region, like an 

 island of trees and shrubs in a sea of hei'baceous prairie species, 

 as to attract the attention of every botanist who passes that 

 way. This is known as Plum Island in allusion to the dense 

 thicket of wild plum that forms the axis of the area. 



The occurrence of this group of woody plants in the midst 

 of a typically prairie flora affords an interesting study of rela- 

 tionships that exist between two dissimilar but contiguous 

 floras. It is well known that the plant covering of any region 

 is more or less unstable. In any plant association there are 

 likely tO' be small changes as species succeeds species or one 

 overwhelms another, but when two such distinct floras as these 

 meet, the struggle is likely to assume more strenuous lines 

 since, owing to the nature of the plants, .they cannot inter- 

 mingle and one must tend to displace the other. The question 

 of how Plum Island came to be, therefore, is one of consider- 

 able interest and a definite answer should throw an interesting 

 light on the problem of whether the forest is succeeding the 

 prairie or the reverse. Is Plum Island increasing in size or is 

 it being surrounded and driven out by the prairie? 



A cursory examination of the region shows that practically 

 all of the woody species in the island are represented on the 

 nearby moraine, and a closer study of the individual species 

 composing it brings out the fact that only eigdit of the her- 

 baceous plants in the island are common tO' the prairie also. 

 The line between the two floras is thus seen tO' be sharply 

 drawn. Of the forty-six species of plants found in the island, 

 more than two-thirds are plants of the moraine. The woody 

 species include the black and burr oaks, the black cherry, the 

 choke cherry, wild plum, wild crab, thorn apple, wild rose, 

 blackberry, red raspberry, black raspberry, common elder, 



