1897] MICROSCOPICAL JOURNAL. 123 



able to understand why it is that their growths occur 

 during- tliose seasons of the year when the water is in 

 circulation throughout the vertical. During- those periods 

 not only is food more abundant, but the vertical currents 

 keep the diatoms near the surface, where there is light 

 enough to stimulate their growth, and wliere there is an 

 abundance of air. If this theory be true, it must follow 

 that the weather has a marked influence u})on their 

 growth. We should expect that the greatest growths 

 would occur oij_ warm, fair days, when there is J ast enough 

 wind to keep the diatoms near the surface. On quiet 

 liays we should expect that they would sink in the water, 

 perhaps below the limit of their growth. During a long 

 period of (piiet weather they might sink even to such a 

 depth that they would not again be able to reach the sur- 

 face. 



This is just what took place in Lake Cochituate in the 

 spring of 1895. In this lake there is almost invariably 

 a heavy spring growth of diatoms, but in 1895 the 

 growth was small, ft began as usual, the diatoms being 

 apparently in good condition. Early in May, however, 

 there were a few days of uncommonly warm weather. 

 The temperatuT-e of the air went above 90°, and tlie tem- 

 perature of the surface water on one day was 7^)°. For 

 almost a week the water was very calm. During this 

 calm weather the diatoms settled rapidly, disappearing 

 almost entirely from the surface. In the meantime tlie 

 water became stratified, on account of the high teiuj)era- 

 ture of the surface layers, and when once more tlu' wind 

 began to blow, its intiuence was felt only ten or fifteen 

 feet below the surface. The diatoms, liaving settled 

 below that depth, wt^rt^ unable to ri.se, and consequentlv 

 their <rrowtli ceased. 



