A STUDY OF RELATIONS BETWEEN PLANT GROWTH. 337 



(4) Note the practical absence of nitrate at the end of winter, and also 

 note the general shape of the nitrite curve. 



(5) Nitrate gradually rose, and then rapidly fell after the plants began 

 to increase. 



The reason for the increase during the winter is of course the fact that the 

 factors tending to increase the content; i. e. the nitrogen content of the in ets. 

 the release of nitrogen to simpler forms by decay, and the nitrogen compounds 

 washed from the air by rain and snow, over-balanced the factors tending 

 to reduce the content, such as loss from the outlet and nitrogen assimilation 

 by plants. The very low nitrite content at the end of winter indicates that 

 the disposal of this form over-balanced its production, at a time when pro 

 duction was slow due to the fact that the cold weather inhibited bacterial 

 action. When the warm weather of spring came, the bacterial action was 

 resumed and the nitrites were again produced faster than they were used. 



About the first of Jime, the phanerogams began to increase rapidly and 

 by the end of July had reached the maximum. This growth although limited 

 to the littoral region clearly affected the pelagic region as is shown by the 

 fact that the results obtained from the open lake were comparable to those 

 obtained where plants grew, except of course the less striking results were 

 found in the open lake, since it acted as a reservoir. It was noticed partic- 

 ularly that the nitrate decreased and came to practically equilibrium at a 

 low level at the time that plant growth was at its maximum. 



The E fleet of Stralificalion. Vertical series were run from Station C, the 

 deepest place in the lake. It was found that the lower part of the lake con- 

 tained more n trogen than the upper. This general fact was noted by Birge 

 and Juday, 1911* in investigations on Lakes Garvin and Mendota, but in 

 other respects my results differed from theirs, due perhaps to differences in 

 the lakes. 



The graph showing the nitrogen content at different levels (Fig. 4) does 

 not represent a single series, but is the average of two or three analyses for 

 each depth. 



This graph offers a partial explanation of the rise of nitrogen content 

 during the winter, since the water mixes from top to bottom during that time 

 due to the holothermous condition, and the water of the bottom being more 

 concentrated causes the surface content to rise. This is insufficient, however, 

 to account for all of the increase since the winter and spring content at the 

 surface is much higher than that at the bottom later in the season at least. 

 Before the spring statifica tion began, the content must have been practically 

 uniform from top to bottom, and the fall to the conditions found when the 

 vertical series was run was probably due to the settling of the albuminoid, 

 and the utilization of nitrates by plants, at least these two were the com- 



*Birge and Juday, Wisconsin Geological and Natural History Survey, 1911. 



