16 OUTLINE OF PLANT GEOGRAPHY 



rapidity, and the quickness with which they take possession of 

 a new country when accidentally introduced by man is sufficient 

 proof of the efficiency of their methods of seed distribution. 



The winged fruits of such trees as the maple, ash and elm, are 

 also adapted to transportation by air currents, but are too heavy 

 to be carried long distances, except under particularly favorable 

 circumstances. 



Water transport is much less common, but nevertheless rivers, 

 especially in flood, may transport masses of vegetation or separate 

 fruits and seeds far from their point of origin. 



Ocean currents also play a not unimportant role in the distribu- 

 tion of seeds and fruits, but this form of transportation is confined 

 mainly to a relatively small number of maritime species. In the 

 tropics, especially, there are a good many species, like the coco- 

 nut, screw-pines, Casuarina, which usually live close to the sea- 

 shore. These strand plants often have fruits which are provided 

 with buoyant and water-proof envelopes which enable them to 

 float in the salt water for a long time, without injury; and such 

 fruits may be drifted for a very long distance before they are 

 thrown on shore, when they quickly germinate. The coconut 

 is the best known example, but there are others like the screw- 

 pines, and certain leguminous species growing on the shore which 

 have fruits adapted to water transport. In the eastern tropics, 

 species of Barringtonia, a handsome tree belonging to the family 

 Lecythidaceae has conspicuous fruits which are often seen floating 



in the sea. 



Perhaps of even greater importance than air currents for trans- 

 porting seeds, is the role played by birds and mammals in plant 

 distribution. The many forms of adhesive fruits and seeds, like 

 "burs," "stick-seed," "cockle-bur," etc; the bearded grains of 

 many grasses, which adhere so tenaciously to the coats of animals — 

 or the trousers of men— are practical demonstrations of Nature's 

 contrivances for the dissemination of her plant children. 



Occasionally there is a sticky matter by which fruits adhere, 

 as in the mistletoe, whose glutinous berries stick to the feet or 



plumage of birds. 



Birds undoubtedly take first place as agents of plant distribu- 

 tion, owing to their rapid flight. Especially in the case of plants 

 with small edible fruits, like many kinds of berries, birds are the 



