DEPARTMENT OF MARINE BIOLOGY. 179 



water at the bottom of the tank rose to 40° C. at the time of full exposure to 

 the sun. The tank was set up on the seventh of June and a final examination 

 was made on the first of July. 



The number of diatoms in the tank increased enormously. In two weeks 

 the floor and sides became brown with them. The appearance of these dia- 

 toms was quite different from those found in normal sea-water. The number 

 of reserve food bodies (metachromatic bodies, volutin grains) in diatoms 

 living in normal sea-water is usually 2, but the diatoms in the tank possessed 

 from 10 to 20 or more of these bodies, and they were frequently connected 

 with each other, forming an irregular, splotchy network. With these diatoms 

 there came up the common marine ameba, Mayorella conipes, in large num- 

 bers. At the termination of the experiment it was found that every watch- 

 glass full of fluid and sediment had a few dozen amebas in it. These amebas 

 were of large size, of very clear protoplasm, with numerous clear crystals and 

 a large number of reserve food bodies; some had one or more diatoms in them, 

 but the great majority did not possess any recognizable food objects. The 

 reserve food bodies were identical in appearance, but not in size or shape, 

 with the reserve food bodies in the diatoms. More of these amebas were 

 found in this culture than in any other that I have ever examined. 



Near the end of the season it was realized that a control tank without 

 potassium nitrate is necessary in order to learn by comparison exactly the 

 effect of the nitrate. Lack of time preventing the setting up of such a control 

 tank, I later staged a similar experiment at Cold Spring Harbor, New York, 

 for the purpose of comparison from a different point of view. I set up a 

 wooden tank of a size similar to the concrete tank mentioned above and ran 

 a stream of water through it at the rate of about 100 liters a day, adding 

 slowly enough potassium-nitrate solution to make the contents of the tank 

 1 part nitrate to 100,000 parts of sea-water. This tank was exposed to direct 

 sunlight from early morning until about noon, when it was shaded by the 

 laboratory building. In this tank also, which was under observation from 

 July 20 until August 16, the diatoms increased enormously in number, cover- 

 ing the sides and bottom of the tank and the small stones in it with a thick, 

 brownish, fuzzy coat. The number of reserve food bodies in the diatoms also 

 increased greatly, up to several hundred in some of them, but the total amount 

 of reserve food material was considerably less than in the diatoms in the 

 tank at Tortugas. Some of the larger diatoms increased the amount of 

 reserve food only two or three times over the normal amount. The overflow 

 from this tank made a small pool in the quartz sand and this pool also became 

 brown with diatoms. In this tank Mayorella conipes and Trichamoeha gumia 

 were found in small numbers. No ameba came up in large numbers. 



No definite observations were made to determine whether the diatoms 

 grown in these tanks possessed shells poorer in silica than those growing in 

 normal sea-water. The very small amount of silica in sea-water has some- 

 times been considered as one of the most important Umiting factors, if not the 

 most important factor, in the growth and distribution of diatoms, and if this 

 were true an excess of nitrate should not produce a marked increase in the 

 rate of diatom growth. As we have seen, however, not onty did the amount of 

 reserve food material increase enormously in the individual diatom, but the 

 number of diatoms themselves, and therefore the number of diatom shells, 

 increased many times over the number usually present in sea-water. 



