THE BASIS OF LIFE IN THE SEA 1 55 



view by the little floating plants called duck-weeds or lemnas. 

 Why do we never find floating sea-weeds living in the same 

 way? On a pond or lake if the duck-weeds are blown about 

 by the winds it does not much matter where they go; the 

 conditions are about the same everywhere and some at least 

 will eventually be washed into a backwater like the one from 

 which they came. In the sea a floating plant if not washed 

 up on some beach would sooner or later be carried to a region 

 with a different temperature or with different chemical con- 

 ditions where it would eventually die, just as the sargassum 

 does. Large free floating plants requiring a large amount of 

 nutritive matter and of sunlight and a more or less constant 

 temperature, at least for considerable periods each year, such 

 as are often so abundant in fresh waters, cannot exist in the 

 sea because of the certainty of eventual destruction through 

 the impossibility of remaining continually within the narrow 

 range of conditions under which alone their existence can be 

 maintained. 



But suppose the bulk of a 400 foot sea-weed were distributed 

 among several billions of microscopic plants. These would 

 soon separate in all directions; some would sink to all depths 

 below the surface, and those at the surface would be widely 

 scattered by the winds and waves. Miflions might be swept 

 away and lost, but other millions would always be present 

 constantly bringing forth millions of young. If small enough 

 and distributed from the surface of the sea down to the limit 

 of effective light penetration, about 650 feet as a maximum, 

 and capable of rapid reproduction, such plants would be 

 unloseable, so to speak, and always permanently present in 

 any given locality. 



This is exactly what occurs in the ocean. The great bulk 

 of marine vegetation we cannot see. It is composed mainly of 

 plants called diatoms, especially prolific in cold regions and 

 at cold seasons, of peridinians in the tropics and at warm 

 seasons, of the exceedingly small coccolithophorids, the very 

 minute flagellates, and of other types. 



