554 Lloyd Williams. — S Indies in the Dictyotaceae. 
tides after the highest. The October crops in the Menai Straits are retarded 
until the original times of initiation and liberation are almost reversed. 
3. Whether liberation in the Menai Straits takes place three tides after 
the highest, or whether it is delayed till the fifth tide, depends primarily 
not upon meteorological conditions, but upon the heights of the spring 
tides. Of the two sets of springs which occur during a month, one is 
generally higher than the other. This alternation of higher and lower 
springs is fairly regular, and, during the fruiting period of Dictyota, is very 
well marked, owing to the exceptional equinoctial tides, the difference in 
some cases amounting to three feet. In the Straits the crop matured 
during the higher springs is liberated about the fifth tide after the highest, 
whereas in the succeeding lower springs the crop is liberated about the third 
tide after the highest. At Plymouth the reverse is the case, the shorter 
interval (seven tides) coinciding with the higher springs, and the longer 
(ten to twelve tides) with the lower spring tides. 
4. The ‘optimum of development’ (= the time taken by an average 
sorus to develop) is variable in length. In the Menai Straits it may be 
from nineteen to twenty-five tides. This inequality is due not to meteoro- 
logical conditions, but to the variable length of the ‘ semilunation,’ or the 
interval between new and full moon, and consequently between the springs. 
Sometimes this is as low as twenty-five tides, while at others it is as many 
as thirty-one. The length of this interval does not affect the actual period 
of liberation, but it determines the length of the ( optimum of development/ 
the average ratio of the latter to the semilunation being 7 6 : 100. 
At Plymouth it is probable that the length of the ‘ optimum of develop- 
ment ’ is directly dependent on the height of the spring tides, and not upon 
the length of the semilunation. 
5. In the Menai Straits, where the crop is started during the waning of 
one set of springs and matured during the succeeding set, development is 
at first fairly rapid ; it then becomes slower during neap tides, finally being 
accelerated again with the increasing ‘rise’ of the spring tides. At 
Plymouth, where the crop starts at or soon after neap, development is at 
first slow, but becomes progressively more rapid towards the close of the 
period. Thus all the facts go to show that development is determined 
by the time and height of the spring tides, and that progress is very slow 
during neaps. 
6 . Owing to this acceleration of development at maturation, the late- 
starting sori at the distal ends of growing plants are not much behind the 
rest of the crop in their liberation, and the ‘ crop-band ’ is consequently 
several tides narrower at the liberation than at the maturation end/ 
7. As the season progresses the crops show greater uniformity in the 
ages of the gametangia, so that the ‘ crop-bands,’ which at first were ten to 
twelve tides wide, become narrowed to six tides or fewer. 
