III. Periodicity of the Sexual Cells in Diciyola. 553 
the plant is only obtainable by dredging, and even then it is so scarce that 
it is very difficult to hit upon. Though excellent tetrasporic specimens had 
been obtained, it was not till August 22 that sexual plants were brought 
up. The gametangia in these specimens showed such uniformity that 
periodicity was irresistibly suggested. All the antheridia were half- 
developed, showing 16-32 cells in surface view, and there were no rudi- 
ments. Scars of one previous crop were disposed in close proximity to 
the midrib, while the current crop area extended laterally outwards, and in 
a proximal as well as distal direction. 
The next successful dredging was on August 29. All the antheridia! 
sori were mature, and there were no rudiments. The female plants had 
numerous mature oogonia, and no young stages ; but the numerous empty 
oogonia with the covers still undissolved indicated very recent liberation. 
On September 1 some more plants were dredged. Of these the male 
plants had all the antheridia completely discharged, and there were no fresh 
sori, while the female plants had the mature oogonia liberated, but showed 
numerous very faint rudiments. The evidence in favour of periodicity is 
very strong, but without additional data it is impossible to decide to what 
extent it conforms with the periodicity of Dictyota. 
While studying Ha User is two additional facts were ascertained, which, 
though not bearing directly on the subject of this paper, are of sufficient 
interest to justify their inclusion 
(1) The antherozoids, as surmised by Johnson 1 , are ciliated, and their 
activity is quite as great as that of the antherozoids of Dictyota. 
(2) The liberated oospheres of Haliseris, when not fertilized, segmented 
a few times parthenogenetically and then died, exactly in the same manner 
as the unfertilized eggs of Dictyota do. 
Summary. 
1. The reproductive period of Dictyota dichotoma in this country begins 
about the end of June or the beginning of July, and continues till the end 
of October or mid-November, the most active months being August and 
September. The antheridia and oogonia are produced in fortnightly crops 
which synchronize with the spring tides. In any given locality the great 
majority of gametangia on all healthy, mature plants are initiated at the 
same time ; they pass through the several stages of development simul- 
taneously, and general liberation and fertilization take place on a particular 
day, at a fixed interval after the highest spring tide. 
2. In the Menai Straits the rudiments appear a few tides before neap, 
and the mature gametes are liberated from three to five tides (i. e. on the 
second or third day) after the highest succeeding spring. At Plymouth the 
crops are later, and liberation does not take place till from seven to twelve 
1 Johnson (’91), Lin. Soc. Journ., xxvii. 
