Ill . Periodicity of the Sexual Cells in Dictyota. 545 
accumulation of reproductive energy and the necessary amount of light 
stimulus to start the ‘ wave ’ of activity. 
By mid-November it generally happens that the antheridial sori 
become very small, and only a few of the cells in the centre of the sorus 
divide up to form antherozoids. This is chiefly due to senility (aided, of 
course, by unfavourable conditions), for one finds even at this time vigorous 
young plants in their first or second crop where this does not occur, and 
female plants with very few unshed oospheres. This shows the importance 
when studying the periodicity of these crops to take into account the age 
and vigour of the individual plant. In the earliest records also there was 
another slight error arising from the fact that when plants whose crops are 
nearly mature are brought into the laboratory, the liberation of gametes 
may be accelerated. In this way one has frequently succeeded in getting 
antherozoids to swarm in the laboratory five tides before liberation has 
started in the sea. How this acceleration is brought about — whether by 
increase of light, a higher temperature, or increased oxygenation — has not 
yet been made out, though several attempts have been made to solve the 
question by direct experiment. 
It has been shown in the case of several plants, particularly of Algae, 
that a mere change of environmental conditions suffices to stimulate them 
to reproductive activity, or to a change in their mode of reproduction. 
It is a point deserving of consideration, whether the alternation of extreme 
conditions experienced during the ebb and flow of the spring tides does 
not in some way have an effect upon sexual reproduction in Dictyota. 
If it has any influence it clearly can be nothing more than a stimulus, 
and the evidences in favour of light being the effective agent in controlling 
periodicity appear overwhelming. In the Menai Straits, where there 
are two periods of full illumination during the day, liberation takes 
place very soon after the highest spring tide ; at Plymouth, where there 
is only one such period, liberation is retarded. In the Straits, when the 
periods of maximum light change owing to the shortening of the day, 
the whole crop period is correspondingly shifted without incurring any 
alteration in its length or any other respect. On the North Wales coasts 
the liberations after low spring tides are earlier than those after high 
springs, whereas at Plymouth the contrary seems to be the rule. In 
all these cases the peculiarities are easily correlated with the times of 
springs and neaps and the consequent effect upon the light. 
I have spoken above of light as controlling the periodicity of gamete 
production, and not as causing it. This is because it is shown in the 
next section that the phenomenon may occur in the total absence of 
tides and of any fluctuation in light besides the usual diurnal one. 
P p 2 
