Light as a Factor in Sexual Periodicity

I OBSERVE with interest the letters of Dr. M. A. H. Tincker and Mr. J. T. Cunningham.¹ In regard to the former, I should like to point out that some observations on the periodicity of plankton diatoms have been made. Only in lakes with a relatively high percentage of dissolved salts do diatoms attain great maxima, and vast maxima have only been found to occur in a few British lakes. Many plankton diatoms occur in the greatest quantity in spring, that is, are reproducing at the fastest rate, but some attain their maxima in summer and autumn. A remarkable fact is that some forms have a double maximum, one in spring and one in autumn. In Lake Windermere the following are known to exhibit this phenomenon: Asterionella gracillima, Rhizosolenia morsa, and others. The maxima in the spring may be explained by light influencing reproduction, whereas other maxima may be due to differences in temperature and percentage of dissolved salts. Although a difficult task, observation of diatom and plankton maxima in the ocean might lead to interesting results.