220 THE REPRODUCTIVE ORGANS 



in which a barren island (about twenty-five miles from 

 the nearest mainland) acquired a new flora and fauna. 



Colonization of a barren island. It was found that the 

 first colonists were microscopic Blue-green Algae, Bacteria 

 and Diatoms, which formed slimy patches on the pumice 

 and ash, and provided a suitable medium in which, later, 

 the spores of Ferns and Mosses germinated. All these 

 bodies are so very minute that they float as dust in the 

 air, and are thus capable of being carried long distances 

 by the wind. In this way spores of these plants were 

 carried from the adjacent islands, and formed the first 

 elements of the flora. Then followed flowering plants 

 having light wind-borne fruits, and along the shore appeared 

 seedlings from seeds carried by ocean currents and washed 

 up by the sea, many of them in logs of wood. The rotting 

 logs brought Fungi, and, in the cracks of the bark, small 

 animals. Birds visiting the island brought other seeds, 

 and lastly, man's influence was seen in the introduction 

 of cultivated plants, and in the v/eeds that followed in 

 his train. In a few years' time, large parts of the island 

 were again covered by rank and luxuriant vegetation. 

 Wind, water, and animals, especially birds, proved to be 

 the three principal agents concerned in carrying the seeds 

 of a new flora to the island. 



In a study of the modes of dispersal of the common 

 plants around us, we find the same agents at work ; and 

 an interesting collection may be made showing the various 

 devices by which Nature secures this end. 



The object to be attained is that seeds should be carried 

 far enough away from the parent plant to prevent over- 

 crowding, and to ensure that they are on suitable ground 

 unoccupied by the same species. We will now study some 

 typical examples of dispersal mechanisms. 



A. Dispersal by wind. The essential condition for wind- 

 dispersal is lightness in proportion to bulk, or a floating 



