II I. Periodicity of the Sexual Cells in Dictyota . 549 
In the first example the irregularity and failure were due to excessive 
insolation and a high temperature, caused by the high level of the 
pool and consequent long exposure. In the case of the bottom plants, 
the same result is brought about by the continual settling of dirt on 
the surface of the thallus. Most of this foreign matter rests upon the 
hair clusters of the plant, and any sharp movement of the plant suffices 
to shake it off. The irregularities caused by the dirt are far greater 
in the lower portion of the thallus, where movement caused by currents 
is less felt. It must be kept in mind that even in these cases the periodicity 
is still sharply marked. 
At Gallows Point, near Beaumaris, there are a number of muddy 
pools where Dictyota grows well. When these are covered at the flow, 
there is an eddy which prevents the current being much felt. The 
result is that the plants are to a great extent deprived of light by the 
mud suspended in the water, and especially by that which settles on the 
surface of the plants, making them appear hoary white. As this dirt 
is mostly kept from actual contact with the surface cells of the thallus 
by the hairs, the injury from mechanical irritation must be very slight 
compared with that caused by loss of light. On the same plant 
might be found, in addition to the normal condition, a large number 
of arrested gametangia, others that were decayed, oogonia which had 
reverted to the vegetative condition and segmented in the manner 
described in my last paper, and still others apparently mature — probably 
belated ones from the preceding crop. In addition to this, we find 
that the plants here are later in starting their reproduction by a fortnight 
or a month, and that they are slower in maturing their crops, than the 
cleaner plants of the Swillies. 
In several places on the coast of Anglesea and Carnarvon, as well 
as in the English Channel, there are quiet sheltered bays where, during 
ordinary springs, there is a sufficient depth of water to protect the plants 
from excessive insolation. During the extraordinarily low water of the 
equinoctial springs, however, the plants are nearly or quite exposed ; 
and as in many of these localities the exposure to maximum light takes 
place at midday or early in the afternoon, the result is that the plants 
suffer injury, the thallus becomes pale, and the gametangia are killed. 
The antheridia are fully divided and appear quite mature, but the an- 
therozoids passively float out. It is probable that one reason for the 
failure of algologists to recognize the motility of the male gametes was 
that the specimens they studied had been injuriously affected in this way. 
Another circumstance that affects the crops unfavourably is the 
presence of other plants within or upon the thallus, and it is in situations 
like the above that this is most apt to be troublesome. Endophytes 
generally grow between the cells, and in some cases their presence is 
