THE AMERICAN BOTANIST 



43 



smooth sumac, red dogwood, silky cornel, hazelnut, wild grape, 

 bittersweet and woodbine and the more conspicuous herbaceous 

 species are Solomon's-seal, two species of smilax, two species 

 of false Solomon's-seal, mandrake, strawberry, ground cherry, 

 and two species of nightshade. These varieties form possibly 

 95% of the individual plants composing the island. Incjuiring 

 into their means of dispersal, the significant fact develops that 

 all bear either berries or nuts and are thus adapted for distri- 

 bution by birds or mammals. By far the greater number are 

 avevectant or bird distributed. 



Returning now, to the question of which flora is encroach- 

 ing on the other, it is apparent that if the prairie had assumed 

 the offensive, we should find more representatives of that flora 

 in the island. Nor would the forest species be represented so 

 overwhelmingly by bird distributed forms. We seem forced 

 to conclude, therefore, that the forest is encroaching on the 

 prairie. Such encroachment might occur at any point where 

 the two floras meet, and evidences that Plum Island is increas- 

 ing its bounds are not wanting, but the establishment of this 

 isolated area in the prairie makes the problem very clearcut and 

 points unmistakably to birds as the principal agents in further- 

 ing the work at this point. 



Plum Island is at least thirty years old. The largest tree 

 in it — a black oak — was recently cut down and showed twenty- 

 six or more annual rings in cross section. There may, of 

 course, have been other trees before it, but if so, no evidence of 

 the fact can be found and all the other individuals in the island 

 are certainly much younger. Reviewing' all the conditions we 

 may, in imagination, recall the series of events that resulted in 

 the occurrence of this group of plants. A line fence parallels 

 the long axis of the island and clustered about this are a 

 number of boulders brought from the adjacent prairie. Doubt- 

 less in the shelter of these boulders young trees sprang up, their 

 seeds brought in by small mammals. Protected in some 



