1895. MICROSCOPICAL JOUHTSTAL 143 



lower strata of water remains stagnant during the sum- 

 mer ; that is to say, tliere are no vertical currents in the 

 water below the depth where the wind ceases to keep 

 the water in motion. In the fall the surface water cools 

 until it reaches the same temperature as the water at the 

 bottom. Then, when the density of the water is the 

 same at all depths, there is a stirring up ; the lower 

 layers are brought to the surface, and the light, lioccu- 

 lent, amorphous matter, always abundant at the bottom 

 when the soil is muddy, is distributed through the water. 

 During the winter, when the surface of the water is 

 frozen, there is another period of stagnation, due to the 

 fact that the temperature of the water at the bottom 

 tends to remain at the point of maximum density (392° 

 F.), while the surface temperature sinks nearly to the 

 freezing point. The winter stagnation takes place in 

 both deep and shallow ponds. 



There are thus two periods of the year, one in the 

 spring and one in the fall, each about six weeks long, 

 when the water is in circulation from top to bottom. It 

 is during these periods that the diatoms develop. 



Study of the physical and chemical conditions of sur- 

 face waters has shown that the two most important con- 

 ditions for the growth of diatoms are a sufficient supply 

 of nitrates and a free circulation of air, and that both 

 these conditions are found at those periods of the year 

 when the water is in circulation. 



As to the effect of temperature on diatams, our obser- 

 vations indicate that the variations of temperature 

 usually met with in the ponds of this climate have com- 

 paratively little direct influence on their growth, cer- 

 tainly not enough to account for their seasonal distribu- 

 tion. Diatoms grow well both in summer and winter, 

 provided food is plenty. Vigorous growths have been 

 observed at temperatures ranging from, 35° to 75° F. 



