372 , THE FRUIT 



germinate. The mistletoe is a parasite on trees, and 

 the modified roots (haustoria) produced by the seedling 

 penetrate the bark and into the wood of the tree. 



(3) Dispersal by Water Carriage. — Many seeds and 

 fruits float when they drop into water, and may thus 

 be carried considerable distances by rivers, germinating 

 if they are stranded in a suitable spot. Many seeds 

 become waterlogged and sink after a short time, but 

 others float for long periods, or indefinitely. The seeds 

 of many riverside plants are surrounded by a tissue 

 containing much air, and are thus specially equipped 

 for floating. 



The seeds and fruits of a large number of species of 

 tropical seashore plants — both trees and herbs — ^have 

 such a floating tissue and are able to stand long soaking 

 in salt water without harm. They are constantly 

 carried about in shore currents and stranded on the 

 coast, where they germinate and produce fresh vegeta- 

 tion. Newly formed islands of coral or volcanic material 

 are largely populated by plants in this way, and this 

 feature of tropical coast plants is also largely responsible 

 for the great uniformity of this vegetation over very 

 wide areas. The coconut' is the best-known example, 

 the very thick woody pericarp being very light in 

 proportion to its bulk and causing the fruit to float 

 indefinitely. 



It has been shown that this character of coastal 

 plants has been developed in different species from 

 other causes not related to the capacity for floating. 

 But it is clear that the species whose seeds and fruits 

 can float will have a much greater chance of successful 

 propagation, and especially of wide dispersal, than if 

 they had seeds or fruits which sank at once or could 

 not long resist soaking in salt water. There is thus a 



