29 



ever, that certain conditions in the environment increase the 

 amplitude of the pulses by hastening the rapidity of reproduction of 

 these organisms. Of the fifteen well-defined pulses appearing in 

 our records of six years, all but three minor ones occur in stable con- 

 ditions, such as pertain to sustained low water. The ""greater part 

 of these pulses, however, occur in declining floods, when contribu- 

 tions from backwaters are considerable. It may seem ill-advised to 

 refer to the conditions of falling river-levels as "stable"; neverthe- 

 less, they are relatively much more stable than those which attend 

 the in-rush of silt-laden flood- waters, and involve fewer changes in 

 factors of the environment. Save in the matter of the relative con- 

 tributions of backwaters and of sewage dilution they resemble 

 those of sustained low water. These Pediastrum pulses are also re- 

 lated to the nitrate pulses (Pt. I., PI. XLIII.-XLV. and Table X.), 

 but the relation is not uniform. In the majority of instances the 

 pulses of 1896-1898 (during which time chemical anaylses are 

 available) coincide approximately with the crest or decline of in- 

 crease in nitrates. For example, the pulse noted on July 17, 1896, 

 of 107,200 from a previous level of 1,210 on June 1, follows a wave 

 of nitrates 'progressing for three weeks and culminating on June 9 at 

 3.25 parts per million arise from 1.5 (Pt. I., PI. XLIIL). On June 

 16 the nitrates have fallen again to 2.2, and on the 23d to 2.0, but 

 rise on the 30th to 2.8. Pediastrum responds to these changes by 

 dropping from 107,200 on the 17th to 15,000 on the 27th, and by 

 rising again on July 2 to 68,400. Not all of the fluctuations in the 

 two are concomitant. Some of the most marked pulses of Pedi- 

 astrum appear at the lowest levels of the nitrates. For example, 

 that of August 30, 1898, of 326,400, follows no nitrate wave, though 

 it coincides with a reduction in nitrates to the minimum of .05. On 

 the other hand, the nitrites had just passed on August 23, an un- 

 usual pulse, to .42, falling again on August 30 to .22 and on Septem- 

 ber 6 to .05 with the passing of the Pediastrum pulse. Pulses of 

 Pediastrum are thus apparently not dependent for their develop- 

 ment upon an abundance of nitrates above the levels shown in the 

 analyses, though a decline in these sources of food or in other forms 

 of nitrogen usually attends these pulses. Pediastrum is but one of 

 many factors among the planktonts, and in the environment, 

 biological and chemical, concerned in these changes, and con- 

 clusive demonstration of its ecological relations must be obtained 



