II L Periodicity of the Sexual Cells in Dictyota. 551 
aeration, and in keeping their surfaces clean ; if, on the other hand, the 
wind is violent, the waves tear the plants away or injure them by abrasion 
of the surface cells, causing various kinds of adventitious growths. When 
Dictyota grows on surf-beaten rocks it is generally dwarf, and in female 
plants most of the oogonia fail to mature, but divide vegetatively until 
the whole surface becomes thickened with parenchymatous growth. 
Why oogonia, when arrested, should sometimes degenerate and 
ultimately fall off, while at other times they revert to the vegetative 
condition and form multicellular growths which remain permanently on 
the thallus, we cannot decide, nor do we know exactly under what 
particular conditions this takes place, except that it is apt to occur late 
in the season, and also in laboratory cultures. It is highly probable 
that in the second case the arrest takes place at an earlier stage than 
in the first. 
Having described habitats where unfavourable conditions prevail, 
it would not be amiss to describe the one where I have found the sexual 
plants to flourish best. This is in the Swillies, a part of the Menai Straits 
already alluded to, where there are deep channels with clean sandy 
bottoms, through which there are continuous steady currents of clear 
water. Immense tufts of Halidrys grow down in the lowest zone, and 
on these tufts an abundance of luxuriant Dictyota plants flourish which, 
in consequence of the clearness of the water, and the currents, aided 
by the continual swaying movement of the long Halidrys on which 
they are borne, are kept perfectly free of both dirt and epiphytes. These 
plants are of a large size, healthy in appearance, and of a rich deep colour, 
while their crops are most abundant and regular in their development. 
On the south coast the habitat most nearly resembling this that has 
come under my observation is in some deep channels to the landward 
side of the Mewstone (Plymouth), where clean and luxuriant Dictyotas 
grow, not upon Halidrys in this case, but upon Cystoseira. 
It has been pointed out that one of the results of unfavourable con- 
ditions is the arrest and sterilization of gametangia. It is interesting to 
note that these are nearly always located at the outer edges of the sori. In 
this way the normally borderless sori of oogonia acquire borders of sterile 
cells, and in extreme cases all the gametangia are arrested, or only a few in 
the very centre arrive at maturity. Even in the case of plants where there 
has been no sterilization the same thing is observable, in the fact that 
liberation generally starts in the middle of the sorus. This localization of 
the reproductive energy is interesting. First there seems a ‘ flow,’ as it 
were, into definite areas which change with each crop ; then we have 
greater vigour in the centre of such an area, as shown in the more frequent 
failure of the peripheral cells. 
While studying the periodicity of Dictyota this summer at Plymouth, 
