543 
III. Periodicity of the Sexual Cells in Dictyota. 
Menai Straits we have ideal conditions for securing to the plants a high 
degree of illumination which at no time is in danger of being excessive. 
Partly as a result of this, Dictyota grows far more luxuriantly and fruits 
more freely and with greater regularity in the Straits than in any other 
locality where I have studied it. 
If the above hypothesis be true, a locality where the times of the tides 
are the reverse of those obtaining at Bangor ought to yield different results. 
At Plymouth, for instance, the low water of springs always takes place 
about midday, thus giving only one period of strong illumination during 
the day. Of the three crops which I have most closely followed at 
Plymouth, two were developed during the higher spring tides of early 
August — one last year and the other at the corresponding period of the 
present year. In both cases general liberation did not take place until 
seven tides after the highest. In the third case, which was a low tide one, 
the crop was still later ; initiation did not take place until after the lowest 
neap, and the crop was not discharged until at least twelve tides after the 
highest spring. The temperature was lower and the light poorer during 
this period than it was during the two preceding ones, so that some portion 
of the retardation may be due to this cause. Even after allowing for this, it 
is clear that all the crops are later at Plymouth than they are in the Menai 
Straits, a result which fits in admirably with the ‘ light J hypothesis. It 
may be suggested that inasmuch as the times of the tides are the reverse 
of those in the Menai Straits, giving two daylight periods of low water at 
neaps and only one during springs, that the times of initiation and libera- 
tion should also be exactly reversed. It must be borne in mind, however, 
that at the spring low water the plants are very much more exposed 
to the light than during the neap ebbs, and, what is still more important, 
that the light at noon is far more intense than at six o’clock. When 
collecting in the vicinity of Plymouth I observed that luxuriant fruit- 
ing plants were never found excepting where they were shaded from the 
direct rays of the noonday sun during the spring ebbs. The various 
forms of Dictyota are very abundant on the south coast, but when they 
are compared with those growing on the North Wales coasts one is at 
once struck with the smaller size of the plants and the greater difficulty 
of finding well-fruited specimens. The luxuriance of the Menai Straits 
plants may be partly due to the greater range of the tides and to the con- 
tinuous circulation of clean sea-water ; but it is highly probable that much 
of the success of the plants there, as compared with those of the south coast, 
depends on the fact that the total illumination obtained by them in the 
Straits is greater than that experienced by the Plymouth plants, and at 
the same time that maximum illumination occurs at a time of the day 
when there is far less danger of excessive insolation. 
It has been pointed out in a preceding section that after the higher 
pp 
