122 TEE AMERICAN MONTHLY [April 



The number of diatoms in the original sample is sliown 

 by the broken line. The point at which this broken line 

 cuts the curves may be called the limit of growth. In 

 Lake Cochituate this point was at a depth of about 

 twelve feet, in Chestnut Hill Reservoir, six feet. 



Diatoms are said to be positively heliotropic, that is, 

 they tend to move towards the light. In some species 

 this power is quite strong; in others it is less noticeable. 

 For the purpose of determining the heliotropism of the 

 diatoms commonly found in water supplies, samples of 

 water rich in diatoms were placed in brass tubes three 

 inches in diameter and thirty-two inches long, having glass 

 ends. One end was covered with a black cap, and the 

 other end exposed to the light. Alter varying lengths 

 of exposure, portions of the water were drawn from each 

 end of the tubes and examined microscopically. As an 

 example of the results obtained the following may be 

 quoted. Cochituate water containing 922 diatoms per cc. 

 was exposed in a tube for twelve hours. At the end of 

 that time the water at the light end of the tube contained 

 1,438 and that at the dark end only 320. Some of the 

 tubes were incliued, to see if the diatoms would move 

 upwards towards the light; some of them were placed 

 vertically; in othei's the diatoms were given time to settle 

 before the exposure was made. The experiments showed 

 that most of the common genera tended to move towards 

 the light while settling, but that having once reached 

 the bottom of the tube they remained where they fell. 

 They api)arently did not possess the power of moving 

 upwards towards the light — certainly not through any 

 great deptli of water. But while they could not rise of 

 their own accord, slight currents of convection caused by 

 varying the temperature of the water sufficed to keep 

 them near tlie surface. 



The bearing which these facts have upon the seasonal 

 distribution of diatoms is obvious, and we are now better 



