DISPERSION OF AQUATIC PLANTS 51 



the sea-water cannot enter the coco-nut, because the outside of 

 the husk is coated by a layer with fatty contents, which prevents 

 the infiltration of water. 



The number of fruits or seeds capable of floating on the water 

 for any length of time is small ; most sink at once, and sooner or 

 later, undergo decomposition at the bottom of the sea. It has 

 been ascertained by experiment that the seeds of several plants 

 can be immersed in sea-water for over a year and yet not lose 

 their power of germination. 



Fresh- water plants, such as the Sedges, Water Plantains, 

 Flowering Rushes, Bur- Reeds are dispersed by the wind blowing 

 them as they float on the surface of the water. The fruit of the 

 Sedge is enclosed in a bag, called the utricle ; this acts as a bladder, 

 the space between the utricle and the nut being filled with air ; 

 by this means the fruit is able to keep afloat, and is blown along 

 from bank to bank, or from marsh to marsh. The seeds of the 

 Arrowhead are not wetted by water, and therefore float ; the 

 fruits of Water Plantains, Bur- Reeds, and other water plants are 

 furnished with a cortical tissue containing a great deal of air 

 that makes them light. 



In the Yellow Water-Lily the walls of the carpels separate 

 into two layers ; the outer one is green and succulent, the inner 

 is white and charged with air, which keeps the fruit afloat. In 

 the white Water-Lily each seed has a coat (arillus) round it, the 

 space between this and the testa is filled with air, enabling the 

 seed to rest on the water until it is driven along by the wind. 



Fruits or seeds which have structures enabling them to keep 

 afloat may be dispersed not only by the wind blowing them along 

 the water, but by adhering to the beak, legs, or feathers of birds 

 which come to the water's edge to drink. Even seeds that do not 

 float, but sink into the mud, may be dispersed in this way, as the 

 mud sticks to the feet of the birds. Darwin's experiments on 

 the number of seeds imbedded in mud gave some very striking 

 results; as many as 537 plants were obtained from the seeds 

 contained in 6f ounces of mud. Now, many birds fly at the rate 

 of forty miles per hour, swallows about one hundred miles an hour ; 

 seeds adhering to them may therefore be dispersed at considerable 

 distances from the parent plant. Kerner in his investigations 



