4] DISPERSAL AND MIGRATION I07 



hyacinth {Eichhornia crassipes), whose dilated leaf-stalks act as floats, 

 as illustrated in Fig. 21, B. It is not, however, by any means neces- 

 sary to float on or drift in the body of water to be water-dispersed. 

 Thus some seeds or fruits sink at first but rise to the surface on 

 germination, to drift until they become stranded — perhaps under 

 conditions ideal for further growth — while many are carried short 

 distances by rainwash or sudden rushes of water over the ground 

 or frozen surface, for example during snow-melt in alpine and arctic 

 regions. Severe floods may dislodge and transport whole trees, 

 as well as innumerable seeds and fruits that are deposited on the 

 wet flood-plain when the water ultimately recedes. Also apt to 

 transport living materials are islands of drifting branches etc., ice- 

 bergs, drifting ice-floes, and the still larger and more lasting ice- 

 islands. These are largely fortuitous and probably capable of 

 involving almost any category of plant from time to time, whereas 

 the regularly water-dispersed plants normally live in or near the 

 water and are modified accordingly. 



The main modes of water dispersal may now be considered : 

 {a) Sea currents. These can cause very effective long-distance 

 dispersal of suitably modified disseminules, in some known cases 

 for over 1,000 miles. For this the body must normally be able to 

 float for a long time without becoming waterlogged and must also 

 belong to a littoral species that can establish itself under saline con- 

 ditions on a sandy, muddy, or other sea-shore. Coconuts are sc 

 dispersed, even if there is some doubt as to whether actual migration 

 is thereby efl'ected ; and among familiar plants of north-temperate 

 and boreal shores that evidentlv migrate in this manner may be 

 mentioned the Oysterleaf {Mertensia maritima agg.) and Sea-beach 

 Sandwort {Arenaria [Hotickenya) peploides agg.). Excellent tropical 

 examples are afforded by the characteristic dominants of mangrove 

 swamps, such as species of Rhizophora and Avicennia, the seedlings 

 of which float widely. Also normally dispersed by sea currents in 

 the manner of seaweeds are further herbaceous maritime Angio- 

 sperms — most often as whole plants, parts of plants, or asexual 

 propagules. On the other hand, the vast numbers of seeds and 

 fruits of freshwater and normal land plants, and of course individuals 

 themselves, that are blown into the sea or carried thereto by rivers, 

 in general perish. 



(b) Rivers and streams. These commonly transport fruits, seeds, 

 and other parts of plants — sometimes as far as from their sources 

 right down to the sea. In other cases they may help with the seeding 



