374 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



Of course many regions are much poorer in diatoms than Kiel 

 Bay, while many are richer; in the Arctic and in the Antarctic they 

 may be so abundant at times as to color the water for miles and to 

 give it a slight, but noticeable, " smooth " feeling. 



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 practically 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 midwinter. 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 others 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 fore- 

 shadowed by the brownish discoloration of the under surface of the 

 ice due to a scum of bottom-living diatoms which have risen up and 

 become attached to it. 



The body of the diatom is inclosed 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 navi- 

 culoid or canoe shaped. Most of the important pelagic diatoms 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 of 

 which in its turn divides into two, making four, and so on. The 

 capabilities of this process may be appreciated when it is realized 

 that if one diatom should divide into two in 24 hours, and its 

 progeny do the same, in the course of a single month 1,000,000,000 

 would be produced. 



So far as I am aware, the rapidity of multiplication of the marine 

 diatoms under optimum conditions has never been satisfactorily 

 determined. But it has been calculated that a single diatom may 

 give rise to 1,000,000,000 in a month. With 6,000,000 diatoms, more 

 or less, to a quart of water in such a locality as Kiel Bay, each one 



