WATER AND MARSH PLANTS 375 



natural sequence, and form a widespread phenomenon. 

 A parallel example, as we have seen, is the invasion of 

 sand-dunes by grasses and sedges, whose very long rhizomes 

 bind and fix the sand and prepare a soil on which a grassy 

 turf or a heath, and eventually a wood, may become 

 established. 



Flowers of water-plants. The flowers of most water- 

 plants are carried above the water, and open in the air. 

 In a few cases they open and are pollinated in the water. 

 This is seen in e. g. the Grasswrack, while some, like the 

 Water Bistort, may be self-pollinated under water, but the 

 flowers do not open, so that really they are pollinated 

 in air and not in water. Usually, however, the flowers are 

 at a disadvantage, owing to the scarcity of insect -pollinators 

 on water, but this is compensated for, as we have seen, by 

 the great powers which water-plants possess of vegetative 

 reproduction. 



Marsh-plants. While there is every gradation between 

 water-plants and marsh-plants, typical examples differ in 

 several respects. The aerial parts of marsh-plants agree 

 closely in form and structure with those of land-plants, 

 and are exposed to similar conditions ; the underground 

 parts grow in wet, cold, badly-aerated soil. In consequence, 

 as in aquatics, some marsh-plants have large air-spaces 

 in their tissues ; others have cylindrical stems and greatly- 

 reduced leaves, e. g. Rushes and Horsetails. Interesting 

 modifications, related to differences in transpiration, may 

 be easily seen in the Meadow-sweet. The lower leaves, 

 which are exposed to little evaporation, are without hairs ; 

 the intermediate leaves have patches of hairs on the lower 

 surface ; and the upper more-exposed leaves have a close 

 covering of silky hairs, which materially reduce transpiration. 



The following is a list of the common plants found 

 growing in a marsh at the head of a small lake which has 

 been invaded by the Pond-weed, Water Milfoil, Water 



