III. Periodicity of the Sexual Cells in Dictyota. 555 
8. Of the various factors (temperature, pressure, aeration, &c.) which 
fluctuate with the alternation of neap and spring tides, the one which seems 
to account most satisfactorily for the facts of periodicity is the increased 
illumination experienced by the plant during low water of spring tides ; 
and the striking difference between the details of periodicity at Plymouth 
and in the Menai Straits are due to the difference in the time of day at 
which low water of spring tides occurs in the two places. 
In the Menai Straits (east end) this always takes place between five 
and six, morning and evening, so that there are two periods of maximum 
illumination every twenty-four^ hours, whereas low water of neaps occurs 
about twelve o’clock. Here there is only one ebb during daylight, and 
the illumination then is much poorer than during low water of springs. 
At Plymouth the reverse is the case, low water of spring tides occurring 
at noon and at midnight ; thus the total amount of light obtained during 
springs is less, and the difference between neap- and spring-tide illumina- 
tion is not so great as in the Straits. The result is that at Plymouth the 
plants are poorer, and the crops several tides later in their development. 
9. Other peculiarities in the times of the crops may be explained in 
the same way, e. g. the retardation of the October crops in the Menai 
Straits is due to the loss of morning and evening illumination owing to 
the shortening of the day. (This retardation is not accompanied by a 
lengthening of the crop period.) The earlier liberation after low springs 
in the Straits is due to the better start obtained by the crops during the 
preceding higher springs. At Plymouth, where each crop, starting about 
neap, coincides with a single set of spring tides, the time of liberation (and 
probably also the length of the crop period) is directly dependent on the 
height of the tides. 
10. The times of initiation and liberation thus fixed primarily by the 
time and height of the spring tides may be slightly accelerated or retarded 
by exceptional meteorological conditions. Winds may cause a difference 
of two to three feet in the height of the water, and a rise of one inch in the 
barometer may result in a depression of six or seven inches in the tide, 
while extreme fluctuations of temperature or light may directly influence 
the rate of development of the crop. 
11. So intimate is the relation between the tides and the crops of 
sexual cells, that a study of the tidal almanac for any locality will enable us 
to predict the actual days during August or September in any given year 
on which general liberation and fertilization of gametes will occur ; and 
even should exceptional conditions prevail, the resulting error will only be 
about a day earlier or later than the one predicted. It follows that in all 
places on our coasts where low water of springs occurs at the same hour 
(i. e. in places with the same ‘establishment’) maximum liberation will 
take place on the same day. 
