1907] HOYT—PERIODICITY IN DICTYOTA 385 
seas and habitats where there are no tides. At the same time, the 
sexual cells are so responsive to changes in the amount of illumina- 
tion that the time of their development in seas where there are tides 
is regulated by the increased illumination obtained during the low 
water of spring tides” (5, p. 548). 
During a stay at the laboratory of the Bureau of Fisheries at Beau- 
fort, N. C., the past summer, a record was kept of the fruiting of the 
sexual plants of Dictyota dichotoma, together with observations on 
the tides and the conditions of growth. The following facts were 
observed. (1) The sexual cells were produced in regular periodic 
crops. (2) The time of the production of these crops bore a definite 
relation to the tides. (3) The crops were borne at monthly instead of 
fortnightly intervals, appearing only at alternate spring tides. (4) 
Rudiments of sexual organs were first observed on the day of, or 
the day before, the greatest spring tide, instead of a few days before 
the least neap as at Bangor. (5) The time taken for the development 
of an entire crop was about eight or nine days, instead of twelve to 
seventeen days. (6) General liberation occurred six days after the 
greatest spring tide, instead of two to five days. (7) Differences in 
the height of different sets of spring tides had no effect on the time of 
liberation or the time taken for the development of an entire crop. 
Thus, the August crop was liberated six days after the greatest spring 
tide, the preceding ebb tides having been the lowest observed during 
the summer. The October crop was liberated at the same interval, 
although the tides for nearly a month previous had been exceptionally 
high at both ebb and flood. (8) Differences in the time intervening 
between one set of spring tides and the next had no effect on the time 
of initiation of rudiments or of liberation of gametes. Although the 
numberof days between one spring tide and the next varied from twelve 
to eighteen, rudiments were always first observed about the day of 
the greatest tide, and liberation always occurred six days after the 
greatest tide of each alternate set of springs. 
A comparison of the record of a single crop at Beaufort, beginning 
with the day when rudiments of sexual organs were first observed, with 
a similar record for a crop on the coast of Wales, will best illustrate 
the difference in development (charts 1 and 2). This record is 
followed in each case with male plants, and the numbers given are 
