548 Lloyd Williams . — Studies in the Dictyotaceae. 
a way as to ‘ remember ’ it, so to speak, after a period of quiescence, 
and under circumstances when the original stimuli were no longer present, 
where the temperature was fairly uniform, and when there was no 
fluctuation in the amount of daylight. This may have some amount 
of truth in it, but it seems to me far easier to believe that the periodicity 
is hereditary : that it arose in some ancestor of Dictyota dichotomy 
and that it was afterwards modelled and fixed by the alternation of 
neap and spring tides until it became what we now see it. Even should 
this explanation be correct, it is difficult, however, to see how the crops 
in the dish should maintain the same optimum length as before, and 
especially how the small fragments should synchronize so remarkably 
after their separation from each other. 
Briefly stated, then, the conclusion we come to is, that the periodicity 
of the sexual cells is a hereditary character, and consequently may be 
expected to manifest itself in seas and habitats where there are no 
tides. At the same time the sexual cells are so responsive to changes 
in the amount of illumination that the time of their development in 
seas where there are tides is regulated by the increased illumination 
obtained during the low water of spring tides. 
Local Conditions and their Effect upon the Crops. 
The most favourable conditions for the production of regular crops 
of gametes are the following : — 
(1) Ample light for their needs, without exposure to much direct 
sunlight at low water. 
(2) A plentiful circulation of pure, clean water. As the plants 
are easily torn away or otherwise injured, the currents or waves must 
not be violent. 
(3) Freedom from epiphytes and from dirt. 
A few examples will show how the crops are affected when these 
conditions are not fulfilled. 
Near the Menai Bridge a very shallow pool, four feet above low-water 
level, had well-grown sexual plants of Dictyota, whose sori were very 
unequal in development, and many of which failed to mature. In the 
channel below, growing on Halidrys, were luxuriant clean plants whose 
sori were all in the same stage, and of which none failed to mature. 
Immediately below these, but growing on the bottom of the channel, 
were other plants which were generally white with mud. At spring 
tide such plants often showed many sori near maturity, but with oogonia 
near their edges, which had been arrested and were degenerating, numerous 
whole sori which had ceased developing, sori that were fairly young, 
together with a large number of germlings growing in situ. 
