Swamps and Temporary Fools 99 



as great as an equal volume of sea water. It may be that the water 

 contained in the tissues of the food and the metabolic water are suffi- 

 cient for the needs of marine birds and mammals ( Clarke and Bishop, 

 1948), or these animals may have some special adaptation for excreting 

 excess salts. 



AMPHIBIOUS SITUATIONS 



It is difficult enough for animals and plants to move from the sea to 

 fresh water or vice versa, but, when aquatic organisms attempt to in- 

 vade dry land, they are flopping out of the frying pan into the fire as 

 far as the water problem is concerned. Nevertheless life on land 

 does have certain advantages. Plants usually find more light, better 

 anchorage, less abrasive action, and more concentrated nutrients. 

 Animals may make use of the more abundant vegetation for food 

 and shelter, the greater oxygen supply, and the possibility of more 

 rapid movement. Against these advantages there is one signal dis- 

 advantage—the scarcity of water. Success in colonizing dry land has 

 been dependent on securing and retaining sufficient water. This 

 could be easily arranged, perhaps, if the organism could seal itself up, 

 but, as already stressed, surfaces must be left open for exchange with 

 the external world. Of immediate importance is the problem of how 

 to feed and to respire in air without drying up. Representatives of 

 relatively few phyla have succeeded in becoming wholly independent 

 of the aquatic habitat. However, some plants and animals have come 

 part way out of the water, or come out for short periods, and we shall 

 consider these first. 



Swamps and Temporary Pools 



Certain organisms have hit upon methods of using the advantages 

 of both the air and the water environment. The emersed vegetation, 

 for example, which grows in swamps or on the margins of lakes has 

 its roots in the water and its upper portions in the air. In this position 

 the roots find plenty of water available and extract nutrients from the 

 mud, while the leaves are in the best position for receiving light from 

 the sun and carbon dioxide from the atmosphere. However, many 

 plants are not able to survive in a partially submerged condition be- 

 cause of inadequate direct supply of oxygen for the roots. Plants 

 that are adapted to existence entirely under water, or with their roots 

 in water or in saturated soil, are known as hijdrophijtes. 



