DEPARTMENT OF MARINE BIOLOGY. 167 



Observations on the Effect of Nitrate on the Growth op Plankton. 



The concrete tank described in my last year's report was set up in the 

 same way as last year and the same quantity of nitrate added. Another 

 tank of the same size was constructed and set up, with the same amount of 

 running water, but without nitrate. Two thicknesses of cheese cloth were 

 stretched over the tanks at a height of about 0.5 meter to shield them from 

 too much direct sunlight. (It was thought last year that the high temper- 

 ature which developed in the tank during midday was inimical to the amebas.) 



On June 30 an examination of the tanks was made. The tank with the 

 nitrated water contained large numbers of brownish flagellates, mostly of the 

 genus Amphidinium. When some of the water was placed in a petri dish it 

 looked slightly brownish. A relatively small number of diatoms were found, 

 only slightly more than in the other tank. About equal numbers of Mayorella 

 conipes were found in the two tanks. These fed mostly on diatoms. Very 

 few flagellates were found in the other tank. 



After examination the nitrate was omitted from the first tank and from 

 now on (June 30) nitrate was added to the second tank. On July 17 another 

 examination was made with the following results: 



In tank No. 1, now without the nitrate: (1) the flagellates had gone down 

 to perhaps one-fourth the number present on June 30; (2) considerable num- 

 bers of very small diatoms had come up ; (3) appreciable numbers of Pontifex 

 horridus, Mayorella conipes, and Trichamceba sphoerarum had appeared. In 

 tank No. 2, now with nitrate in the water: (1) about three-fourths as many 

 flagellates were found in this tank as in No. 1; (2) very few diatoms were 

 present; (3) large numbers of Mayorella conipes were present but no other 

 amebas. These amebas fed on the flagellates. 



The addition of nitrate to the water evidently caused an appreciable 

 increase in the number of flagellates, but the effect was not so striking as in 

 the experiments last season upon the diatoms. That the diatoms did not 

 come up this season and the flagellates not last season may have been due to 

 their rarity or absence from the Tortugas waters in the respective seasons. 

 No special search was made for the flagellates in other seasons, but they were 

 certainly not as plentiful during the past three seasons as they were this 

 season; and this season the large diatoms which were so plentiful in the tank in 

 1922 were almost if not entirely absent from my cultures. (The necessity for 

 a careful search was not appreciated until a few days before I left Tortugas.) 

 Further experiments will show by comparison with those already performed 

 what the effect of an excess of nitrate is on flagellates and diatoms and indi- 

 rectly on the larger amebas. 



Observations on Spiral Movements of the "Atlantic Palolo" and on Salpa. 



The study of spiral movement in organisms generally is an outgrowth of 

 my observation in 1917 that amebas move in sinusoidal paths when free to 

 do so. Observations have been collected on many animals and motile plants 

 since that time, and it appears now that the tendency to spiral movement is 

 universal among organisms and that the explanation of this tendency can 

 best be obtained by continued observation on as many different small groups 

 of organisms as possible. 



The "Atlantic Palolo" (Laodice fucata) swarmed on July 7 this year. 

 The general observations of Mayor published in 1908 are confirmed here. 

 The detached posterior (sexual) end of the worm swims about, rotating to the 



