REPRODUCTION AND DISPERSAL 923 



seminules move in definite directions, as in rivers and in the better- 

 defined ocean curren s, but in ponds and lakes the direction of move- 

 ment commonly varies vsfith the winds. All seeds are heavier than air, 

 and hence are incapable of indefinite propulsion in that medium, but 

 many seeds and fruits are lighter than water, and hence may be carried 

 for great distances; among the latter are the fruits of many water plants 

 and swamp plants, such as Sagittaria and Sparganium, whose lightness 

 is due largely to the presence of prominent air chambers in the pericarp 

 or testa. Many seeds, however, sink in water, some rapidly and others 

 more slowly, so that the distance they may traverse is more or less limited, 

 as with wind-scattered seeds; among the seeds which sink at once are 

 included those of such pronounced hydrophytes as Ceratophylluni and 

 Subularia. 



Of great significance in connection with water dispersal is the degree 

 of resistance to the entrance of water offered by floating seeds and fruits. 

 Many seeds capable of floating soon lose their vitality through the en- 

 trance of water, which thus institutes decay. Particularly is this the 

 case if the water is rough, and more particularly if it is salt as well as 

 rough. For example, the coconut, whose fruit often is seen floating on 

 tropical seas, loses its vitality within a few days through infiltration, so 

 that it is doubtful if it could populate a new land at a great distance, 

 though no illustration of water dispersal is quoted more frequently. 

 In contrast with the coconut are such fruits as that of Suriana maritima, 

 a common plant of tropical strands; these have been shown experimen- 

 tally to be uninjured after floating for 143 days in rough salt water, and 

 the seeds of Hibiscus tiliaceus similarly have been shown to be capable 

 of floating for 121 days without injury. The presence of air chambers, 

 especially in the pericarp, greatly retards water infiltration. In Barring- 

 tonia the resistance to infiltration is so great that broken pieces of the 

 fruit float for more than twenty weeks in a 3 per cent salt solution. The 

 seeds of Asparagus may retain their vitality when soaked in water for 

 a year, and in many water plants (as Sagittaria and Proserpinaca) 

 the seeds may retain their vitality at the bottom of ponds for several 

 years. It can hardly be doubted that in all cases the retention of vitality 

 in immersed seeds is due to the resistance of the various coats to infiltra- 

 tion. 



Dispersal by animals. — Many fruits, mainly indehiscent, are scat- 

 tered involuntarily by animals, particularly the bur fruits and others with 

 hooked appendages. Unpleasantly familiar fruits of this character are 



