III. Periodicity of the Sexual Cells in Dictyota . 541 
had only been verified for the coasts of North Wales, obtained also in 
other districts, or was merely a phenomenon induced by circumstances 
peculiar to the locality. A number of short visits had been paid to 
Weymouth, Torquay, Sidmouth, and other places, and the sexual plants 
observed on these occasions clearly suggested periodicity. During the 
summer of last year, as well as of this, I spent five weeks at the Marine 
Biological Laboratory in Plymouth, and while there I was able to follow 
out the whole course of development of a number of crops. The periodicity 
is as well marked here as in the Menai Straits, but the liberation of the 
crops is delayed several tides beyond the time when it occurs on the North 
Wales coasts. This is exceedingly significant, and the reason for it will be 
discussed later on. 
Furthermore, a careful examination of the plants in the herbaria of 
the Natural History Museum and of Kew shows clearly that periodicity 
occurs on the coasts of other countries, especially on those of France and 
Spain, and even in various parts of the Indian Ocean and Australasia. 
Besides, there are unmistakable indications of its obtaining in the case 
of several foreign species of Dictyota. At the same time specimens 
were seen where sori of different ages were mixed up together. It 
would not be wise to generalize too much from herbarium specimens, but 
we may safely conclude that periodicity obtains generally in the case of 
several species of Dictyota in seas where the rise and fall of the tide is 
fairly well marked. 
Probable Cause of Periodicity. 
Whatever factor be suggested as the cause of this phenomenon, it must 
of necessity be one which varies with the tides and which produces its most 
potent effect during or soon after the spring tides. There are several which 
seem to satisfy these requirements to some extent, such as variations in the 
degree of aeration and of pressure, and particularly differences of tempera- 
ture and of illumination. 
The amount of aeration may possibly have some influence on the form 
and on the size of the plants, but it is most unlikely that it has anything to 
do with determining the periodicity of reproduction, for the great difference 
in amount of aeration between deep-growing plants never exposed, and 
those which are habitually emergent during low water of spring tides, is not 
reflected in any way in the condition of the sexual crops. 
The alternation of pressure may have a much greater influence as a 
stimulus than we generally give it credit for, but actual observation of the 
widely different kinds of habitats in which Dictyota flourishes — in rock- pools 
at half-tide level as well as in deep channels, some never exposed and others 
left bare by the majority of spring tides — persuade us that this factor does 
not satisfactorily explain the phenomenon. 
