542 Lloyd Williams . — Studies in the Dictyotaceae. 
As for temperature, though it is a fact that exposure to a very high 
degree of heat brings about irregularities in the development of the 
gametangia, or otherwise causes injury to the plants, it is extremely 
improbable that the particular fluctuations of temperature to which the 
plants are subjected as a direct consequence of the alternation of neap and 
spring tides, are the effective causes of the regular succession of crops. If 
we take the Menai Straits, for instance, the difference between the tempera- 
ture of the water in the channels where the current is nearly always flowing, 
and that of the shallow bays and eddies, or the pools left by the retreating 
tide, is enormous. A few inches below the surface the temperature in the 
current is fairly constant, showing a mean of about 13 0 C. in June, 14-5° in 
July, and 15*5° in August. When plants are left in shallow rock-pools 
they may in a short time experience a temperature of 21 0 C. or more ; 
and as for the plants that are left quite uncovered, they will be subject 
to extremes of temperature that are greater still. It is important to 
remember that the most luxuriant plants and those which display the 
greatest regularity in the development of their crops are those which grow 
in the deep channels, where the temperature shows the least amount of 
fluctuation. 
The lowering of temperature during October and November may have 
something to do with the retardation of the crops, but only as a secondary 
cause, as will be shown below. 
I have been definitely forced to the conclusion that the stimulus which 
incites the plants to produce their fortnightly crops of gametes is the 
periodic change in the amount of light received by them, as a direct result 
of the alternation of neap and spring tides. 
The time and mode in which the rhythmic variation of light takes 
place varies in different localities according to the time of the tides. Thus 
in the Swillies (Menai Straits) low water of the highest spring tides always 
takes place about six o’clock, morning and evening, while that of the lowest 
neap tides takes place about noon. The result is that during low water of 
neaps a considerable depth of water covers the plants, and this intercepts 
much of the light (Diagram V). It would be instructive to measure the 
amount of light obtained by a plant at various times ; hitherto there has 
been no opportunity of doing this. During low water of spring tides many 
of the plants are actually emergent, and the others are so near the surface 
that they suffer but little loss of light. This is shown in Diagram VI. But 
this is not all, for during neap tides only one low water period occurs 
during daylight, the other taking place about midnight, whereas in the 
spring tide periods there are two exposures to the maximum of light — one 
at six in the morning, the other about the same hour in the evening. If, 
as is here contended, light be the determining factor in bringing about 
this rhythmic succession of crops, then it is perfectly evident that in the 
