1907] HOYT—PERIODICITY IN DICTYOTA 389 
behavior of this alga at Bangor and Plymouth may be explained by 
the assumption that illumination is the determining factor, the facts 
observed at Beaufort indicate that for the plants of this region, at 
least, some other explanation must be sought. That the time of 
fruiting is determined to some extent by the tides is indicated by the 
very exact relation which this bears to the tides. That illumination 
is not the determining factor is indicated by the facts that fruiting 
occurred only on alternate spring tides, that differences in the amount 
of light as determined by the state of the weather and the heights of 
different sets of spring tides had no effect on the time of fruiting, and 
that the differences in light conditions to which individual plants are 
subjected are greater than those differences to which all the plants 
are subjected because of differences in the height of water at spring 
and neap tides. 
As was mentioned above, the average difference in the height of 
low water at Beaufort at spring and neap tides is only 0.5 foot. As 
the difference in the depths at which individual plants grow is at least 
six times as great as this, the difference in the amounts of light received 
by different plants at any instant is greater than the differences in 
the amounts of light received by any one plant at low water of spring 
tide and at low water of neap tide. And yet all the sexual plants 
observed in all situations fruited at the same time. It may be urged 
that the sexual plants are sensitive, not to the actual amount of light, 
but to the maximum intensity, and that this occurs at the time of the 
spring tides for the plants of all situations. But the same reasoning 
applies to all the other factors which are influenced by the tides— 
such as aeration, pressure, etc.— so that there is no more reason on 
this ground to ascribe the behavior to the light than to any one of the 
- other factors. 
One observation is interesting in this connection. A female 
plant was brought into the laboratory August 11 and placed in a 
covered jar of sea-water near a window. As is often the case when 
Dictyota dichotoma is subjected to unfavorable conditions, prolifera- 
tions were formed from the margin of the thallus. When examined 
October 6, most of the plant had died, but several proliferations were 
still alive, and these showed sori of uniform age only slightly less 
advanced than those on plants growing in the harbor. In this case 
