THE PLANTING OF OUR FLORA 163 



keyensis, Exostema caribaum, the Jamaica dog- 

 wood, and several others. As these streams all 

 rise in the Everglades and empty into the sea the 

 plants cannot migrate around them. The water 

 alone does not prevent their passing, but the low 

 hammock and swamp on each side of it may do 

 so. 



To this it may be objected that as the birds eat 

 the drupes and berries of many of our hammock 

 trees and shrubs, ejecting the seeds undigested, 

 the watercourses could form no barrier to their 

 flight. But I have found that in almost every case 

 where the streams seem to limit the distribution of 

 plants their seeds are not such as would be carried 

 by birds. Those of the dogwood are winged; of 

 the crabwood, Bejaria, Ceratiola, and some others 

 are contained in capsules and the Lysiloma bears 

 beanlike seeds in pods. Probably nearly all of 

 these are eaten by birds but the seeds are of the 

 digestible sort. 



Of course climate acts as a check to the northern 

 or southern distribution of many forms, there 

 being a limit of heat or cold which they cannot 

 endure, and these climatic boundaries seem to be 

 sometimes coincident with the watercourses. 



