18G PLANT COMMUNITIES 



they can be carried in that medium is necessarily limited. ]\Iany 

 seeds and fruits, however, are lighter than water and so may float 

 almost indefinitely. This is especially true of the fruits of many 

 water plants which often have large air spaces in the ovary walls or 

 the seed coats. The seeds of many land plants, however, sink in 

 water, some quickly and others after a longer time, so that the dis- 

 tance to which they may be carried is limited. The length of time 

 that a seed may float on water and still be viable is closely connected 

 with its resistance to the infiltration of water. Many seeds that are 

 capable of floating rather quickly lose their ability to germinate be- 

 cause of the entrance of water which institutes decay. This is 

 likely to be especially true if the water is rough and still more so if it 

 is salty as well as rough. For example, the cocoanut, which is often 

 seen floating on tropical waters, has frequently been cited as a 

 typical example of water dispersal to distant islands. This fruit, 

 however, usually loses its viability within a few days through 

 the infiltration of water so that it is doubtful whether it could 

 populate a very distant land unless it were carried on driftwood or 

 by some comparable means. On the other hand, there are some 

 fruits and seeds that can float on water for a very long time without 

 becoming injured. The fruits of Suriana maritima, a common 

 tropical plant, have been shown by experiment to be uninjured after 

 floating on rough, salty water for one hundred and forty-three days, 

 and the seeds of Hibiscus tiliaceus were viable after floating in a 

 similar way for one hundred and twenty-one days. The seeds of 

 some species of Asparagus may be viable after soaking in water for a 

 year, and those of the arrow-leaf {Sagittaria) and of the mermaid- 

 weed (Proserpinaca) have been shown to be uninjured after soaking 

 at the bottom of a pond for seven years. 



Fruits and seeds are often carried great distances on driftwood or 

 other articles floating on water. Furthermore the seedlings of some 

 kinds of plants may float for days on water and then, if stranded in a 

 suitable place, take root and grow. The seedlings of some species 

 of bur marigold {Bidens) are sometimes seen floating on water by the 

 hundreds and many of them later succeed in finding suitable places 

 to grow\ In the case of duckweeds and many algae the mature plants 

 float on water and may be carried from place to place. 



118. Migration Through the Aid of Animals.— ^Nlany indehiscent 

 fruits, especially bur fruits and others that have hooked appendages, 

 are involuntarily distributed by animals. The cocklebur 

 {Xanthium), burdock (Arctium), bur marigold {Bidens), hound's 



