1gto] - LEWIS—PERIODICITY IN DICTYOTA 63 
the surface, and the difference between the height of low water at’ 
spring and neap tides is only 0.25 feet. This slight difference can 
hardly cause any considerable variation in the total illumination of 
the plants under consideration, not as much as is caused by alter- 
nating cloudy and sunny days. It is interesting to note, however, 
that both initiation and liberation occur at Naples on the day that 
low water occurs at or nearest midday. Thus low water at midday 
occurred at Naples on March 29, April 10, and April 27, 1908, the 
days when initiation and liberation were found to occur. Whether 
this is more than a coincidence is still to be seen, but the fact remains 
that the critical points in the sexual life of Dictyota coincide exactly 
with the periods of maximum intensity of illumination. 
If one considers the behavior of Dictyota at Naples alone, it seems 
a fairly satisfactory hypothesis that the effective factor in producing 
periodicity is the stimulus of the maximum intensity of light. When 
one comes to apply this hypothesis to the other regions where Dictyota 
has been studied from this standpoint, it becomes evident that, if 
true at all, the hypothesis is modified by other factors. At Beaufort. 
for instance, low water at midday occurs two days before new and 
full moon, while initiation occurs on the day before or the day of 
the greatest spring tide. At Bangor, on the other hand, low water 
at midday occurs about five days before new or full moon, and initia- 
tion takes place one to two days before the least neaps. At Plymouth 
low water of the greatest spring tides occurs at midday, and here the 
times of initiation and liberation coincide more nearly with the periods 
of neap tides. It is obvious, then, that the simple explanation that 
might suffice for Naples is not sufficient for other localities, and that 
the operative factor or factors must be sought by further investi- 
gation. 
It seems possible that Dictyota, in adapting itself to differing 
conditions at various localities, has acquired its habit of periodicity 
in response to different factors. That a similar stage in the repro- 
duction of other algae may be induced by different stimuli has been 
shown by FrEuND (1) to be the case in Oedogonium and Haema- 
fococcus. In these forms the external conditions leading to the 
formation of zoospores differ according to the condition of the indi- 
vidual plants. For instance, cysts of Haematococcus form zoospores 
