THE BASIS OF LIFE IN THE SEA 1 57 



The preceding paragraphs would seem to imply a static 

 condition in the oceanic flora, which is quite the reverse of the 

 real condition. The calculation given for Kiel Bay is for a 

 single season only. In a given spot at times the sea may swarm 

 with microscopic plants, while at other times it may be practi- 

 cally barren. In some places the difference in productivity 

 at different seasons is almost as great as the difference in the 

 productivity of your garden between mid-summer and mid- 

 winter. Usually the difference at different times is less than 

 this, but the amount of plant life present in any given region 

 of the sea is always very variable. 



What are the diatoms? The diatoms are very minute 

 plants which occur wherever there is moisture and light, in 

 fresh, brackish and salt water, on the moist surfaces of rocks, 

 etc. The fresh water forms all differ from those in the sea 

 or in brackish water. Many kinds live attached, but many 

 float about suspended in the water, often in incredible numbers. 

 The attached forms usually form a brownish stratum or a furry 

 covering over plants and other objects in the water. In the 

 Arctic the beginning of spring is foreshadowed by the brownish 

 discoloration of the under surface of the ice due to a scum 

 of bottom Hving diatoms which have risen up and become 

 attached to it. 



The body of the diatom is enclosed within two lids or valves 

 which fit together somewhat like the bottom and cover of a 

 pill-box. These are fashioned of silica, and are of the most 

 exquisite beauty, often highly ornamented, and of various 

 shapes, oval, crescentic, S-shaped, linear, or wedge-shaped, 

 though most of them are naviculoid or canoe-shaped. But most 

 of the important marine types form chains. Of a medium sized 

 species it would take 200 individuals in a row to make an inch; 

 while a few are larger than this, many are much smaller. 



Diatoms reproduce mainly by simple division into two, each 

 one which divides into two, making four, and so on. The capa- 

 bflities of this process may be appreciated when it is realized 

 that if one diatom should divide into two in twenty-four hours, 



