AMPHIBIOUS PLANTS 



47 



to the plants on their banks, but also by the debris and humus 

 they deposit in their course. A river that is flowing at the rate 

 of only three inches per second will bring down fine mud ; a 

 velocity of six inches per second supplies enough force to carry 

 down fine sand ; twelve inches will move fine gravel along ; twenty- 

 four inches will roll along rounded pebbles an inch in diameter, 

 whilst a velocity of three feet is required to move stones about 

 the size of a hen's egg. Rivers flowing through calcareous dis- 

 tricts carry down a great deal of matter dissolved in the water ; 

 it is estimated that the Thames carries down no less than 450,000 

 tons of salts in solution annually. 



AMPHIBIOUS PLANTS 



Some few plants are able to live equally well in water 

 as on land ; they are sufficiently plastic to adapt themselves 

 to either medium. The best known 

 instance of this is the Polygonum 

 amphibium, which flourishes equally 

 well on the top of a bank or in ten 

 feet of water ; but the character of the 

 plant varies with the habitat. When 

 growing in dried-up ponds, or muddy 

 ditches, the stems are creeping at the 

 base, then shortly erect, and the leaves 

 are often downy. On the top of a 

 bank the plant sends up erect leafy 

 stems, usually without flowers. In 

 water, the long stems reach the sur- 

 face ; the thick, leathery leaves, three 

 to six inches long, float on the surface 

 of the water, and dense spikes of rose- 

 coloured flowers project out of the 

 water. The Polygonums are closely 

 allied to the Docks, but are less stiff 

 plants, and some species are creeping 



or floating. The Water-Pepper Poly- 

 Pic. 17. Amphibious Polygonum 



gonum is a slender plant, sometimes (Polygonum 



