1907] HOYT—PERIODICITY IN DICTYOTA 391 
coast (about six inches), this condition, if true, supports WILLIAMS’ 
conclusion that “the periodicity of the sexual cells is an hereditary 
character and consequently may be expected to manifest itself in 
seas and habitats where there are no tides.” 
As the Jamaican specimens of Dictyota belong to more than one 
species, the facts indicate the probability that periodicity obtains 
throughout the genus. The apparent correspondence in the time of 
fruiting of the Port Antonio specimens with those of Beaufort, and of 
the Port Morant specimens with those of Bangor may be only a coin- 
cidence. It is certain, however, that the time of fruiting of the Port 
Morant specimens did not coincide with that of Dictyota at Beaufort. 
Evidently we need more observations before we can draw general 
conclusions or suggest explanations of the facts already observed. 
The author is able to verify WILLIAMs’ statements that light is not 
necessary for the escape of the sexual cells of Dictyota dichotoma, 
and that the tetrasporic plants show no periodicity. 
From a single set of observations, WILLIAMS gives evidence of 
periodicity in ‘‘ Haliseris’’ and suggests by inference the possibility 
of its occurrence throughout the Dictyotaceae. A species of Padina, 
probably P. Durvillaei Bory, is abundant at Beaufort in the same 
situations with Dictyota. Numerous male plants were found and 
these showed no evidence of periodicity, sori of all ages occurring on 
the same plant. It seems, therefore, that periodicity is not character- 
istic of all the Dictyotaceae. All the sexual plants of Padina which 
were examined bore only antheridia and produced these on both 
sides of the thallus, differing in these respects from the printed descrip- 
tions. The antheridia were numerous, being borne in sori in wide 
concentric rows which appeared white to the naked eye, and the sori 
were usually produced in regular acropetal succession. 
The author expects to continue this work, but, believing the 
present data to be insufficient to explain this periodicity, gives the 
results already obtained with the hope of inducing other botanists 
who are favorably situated to begin a study of this form. It seems 
that only by observations of the fruiting and careful comparison of 
the conditions at a number of places for a considerable period of time 
can we hope to understand this striking phenomenon. 
Indebtedness is gratefully acknowledged to Hon. Grorci: M. 
Bowers, U. S. Fish Commissioner, for the privilege of working in 
