LIFE IN THE OCEAN CLARK 373 



On pools and ponds and in quiet backwaters from lakes and rivers 

 in the summer time the water is often quite hidden from view by the 

 little floating plants called duckweeds or lemnas. Why do we never 

 find floating seaweeds living in the same way? On a pond or lake, 

 if the duckweeds 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 abund- 

 ant in fresh waters, can not 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 seaweed 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. Millions 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 ma- 

 rine vegetation we can not see. It is composed mainly of plants 

 called diatoms, especially prolific in cold regions and at cold seasons, 

 of peridineans in the Tropics and at warm seasons, of the exceed- 

 ingly small coccolithophorids, the very minute flagellates, and of 

 other types. 



The numbers of these little plants can only be imagined, not really 

 appreciated. It has been calculated that in the water of Kiel Bay 

 there are 6,336,000 diatoms alone per quart — or were at the time the 

 calculation was made. If there are 6,336,000 diatoms in a quart of 

 water, how many would there be in an area of the ocean the size of 

 the State of Rhode Island, that is, 1,250 square miles, down to a 

 depth of 650 feet, the depth to which at least they may be assumed 

 to live? 



1454—25 25 



