544 Lloyd Williams. — Studies in the Dictyotaceae. 
springs the crops in the Menai Straits are later in their liberation by at 
least two tides than after low springs. The idea that this apparent 
retardation is due to excessive illumination, or to exposure to the air during 
the extraordinary low water of these tides cannot be correct, for it obtains 
in deep channels and in well-shaded rock-pools just as in more exposed 
habitats. The explanation is to be sought for at the other end of the crop 
period. A low spring crop liberates early simply because it had a good 
start during the higher springs of the preceding series, and the comparative 
lateness of the high spring crop is due to the less favourable light conditions 
prevailing during its early stages. 
In the case of Plymouth it is highly probable that the above relations 
are reversed. The two higher springs recorded agreed in having the libera- 
tion seven tides after the highest, while the low spring crop took as many 
as twelve tides before the plants were cleared. When the records are so 
few it is unsafe to generalize, but if the above relation be correct it will be 
seen that this also harmonizes perfectly with the hypothesis. In these cases 
the crop periods lie wholly within the respective spring tide periods. It is 
only natural then that a high spring crop should mature early. It is in 
accordance with this also that we find the optimum length in the latter 
shorter than in the lower springs. 
The remarkable retardation of the crops in the Menai Straits during 
October can also be easily explained in the same way. That this reversal 
of the time is not due to very unfavourable circumstances is shown by the 
fact that the optimum length of the crop is not increased. As the day 
shortens, the light during both morning and evening of spring tide ebbs 
becomes gradually poorer, until at last the midday light of neaps is actually 
better than morning and evening light of springs. The result is that the 
crop periods very nearly correspond to those obtaining at Plymouth. 
A very striking instance of this is also seen in Cardigan Bay, where 
low water of spring tides takes place about three o’clock. Here, during 
August and September, sexual plants are far more liable to injury by 
excessive insolation than are the Straits plants, while later in the season 
the light during the early morning hours is missing altogether. These two 
peculiarities combine to bring the fruiting period of the plant to a close 
much earlier than in the Straits. 
In August and September there is an overlap of the crops amounting 
frequently to two or five tides. During October, however, there is a 
gradually widening interval between the clearance of one crop and the 
first appearance of the succeeding one. A very remarkable circumstance, 
however, is that in spite of the retardation in the time of initiation there is 
no lengthening of the whole crop period. This seems to show that the 
rhythmic influence is so fixed in the plant that when conditions of light 
(and of temperature) change, the new crop is delayed until there is a sufficient 
