481 



to the ends of the months it will be possible, for the purpose of 

 utilizing our data in monthly totals and means, to divide the 

 year into two periods, April-September and October-March, of 

 greater and less illumination respectively. The contrast in 

 illumination is further heightened by the fact — derived from the 

 following table of cloudy days — that the number (at Havana, 

 159) of cloudy days between the vernal and autumnal equi- 

 noxes is only about one half that (311) between the autumnal 

 and vernal. On the average, the season of greatest light is also 

 the season of greatest production. Thus, in the channel waters 

 average monthly production in April-September (4.76) is seven- 

 fold that in October-March (.67), and in the backwaters, such 

 as Quiver, Thompson's, and Phelps lakes, it is respectively 5-, 

 2.2-, and 1.6-fold greater. The records of individual years in 

 all of the localities will be found to exhibit a similar relation- 

 ship. We may infer, accordingly, that the increased light be- 

 tween the vernal and autumnal equinoxes tends to increase 

 production, and that the decreased amount in the remainder of 

 the year tends to lower it. It operates, of course, in conjunc- 

 tion with other factors, and our records contain not a few in- 

 stances where production in the period of less illumination ex- 

 ceeds that in the period of greater light. For example, on 

 December 20, 1898, in the minimum illumination of the year 

 and under an ice-sheet 21 cm. thick, which still further reduced 

 the light, the plankton production in Thompson's Lake reached 

 an amplitude (2.58) exceeding that on June 21 (2.47) in the 

 same lake in maximum illumination, an amplitude, moreover, sur- 

 passed but once from June to October. Other factors are thus, at 

 times, at least, more potent than light in controlling production. 

 A phenomenon of like import exists in the conditions of 

 illumination and production in Quiver and Thompson's lakes. 

 Both lakes are of approximately the same depth, but the 

 former, especially in low-water conditions, has remarkably clear 

 water, the bottom being generally visible, while the latter is 

 always more turbid, and light penetrates the water far less 

 completely. Nevertheless, the lake, with most illumination, 



