41 6 THE BIOLOGY OF FLOWERING PLANTS 



and in the number of species affected. Fresh-water plants 

 have often a very wide distribution, but this is probably 

 in the main due to bird dispersal, and not to the agency of 

 the water. 



fi'i Dispersal by ocean currents is much more important, 

 though it is also limited almost entirely to the carriage of 

 strand and other shore plants, in particular those of the 

 tropics. It has formed the subject of much investigation, 

 especially by Schimper (1891) and Guppy (1906,1912, 19 17). 

 Perhaps Darwin was the first to realise the fact that many 

 seeds and fruits could float for long periods undamaged in 

 salt water. Seeds and fruits may reach the sea through 

 rivers, floating independently, or wedged in crevices of 

 logs, but it is certain that inland plants, even when they are 

 transported by ocean currents, are practically never cast up 

 in a situation in which they can maintain themselves. The 

 seeds of strand and mangrove plants are frequently carried 

 for hundreds and thousands of miles before being thrown 

 up, and after such voyages they often germinate and may 

 become estabUshed. At least forty species have been 

 carried to Krakatau by ocean currents. 



An instructive case is afforded by the floras of the two 

 types of mangrove forest. The Eastern Mangrove has a rich 

 flora very uniform along the coasts of East Africa, India, 

 and Malaya. The Western Mangrove has a poor flora, the 

 important species being the same on the west coast of 

 Africa and the east coast of tropical America. The two 

 types have no species in common. Distribution throughout 

 the two regions has been entirely by ocean currents, Guppy 

 states, for the mangroves and their associates of the western 

 region {Rhizophora Mangle, Avicennia nitida^ Laguncularia 

 racemosa, Anona palustris, Carapa guaianemis), that all are 

 capable of floating in sea water for at least two months, and 

 that all could be carried by the main equatorial current from 

 West Africa to Brazil. The case of the mangroves is of 

 special interest, as the fruits are viviparous, and it is the 

 germinating seedling which is carried. 



Curiously, the fruit which has for long been the type 



