53 2 Lloyd Williams. — Studies in the Dictyotaceae. 
regular in its periodicity, but towards the end of October it becomes slower, 
finally ceasing about the middle or end of November. In very mild and 
bright winters new sori may be produced even at Christmas ; this, however, 
is exceptional, and the sori sometimes seen on the plants in mid-winter are 
in most cases arrested ones from October. 
The proximal portion of the thallus as well as the edges are always 
free of sori ; and when a plant is in its earliest crops and still actively 
elongating, the distal parts are also without reproductive cells. The 
remaining portions have on both surfaces numerous sori, which are all of 
the same age, except a few at the distal end of the reproductive area, which 
are always younger. The reason for the latter fact is evident : this portion 
of the thallus being younger and only just removed from the meristematic 
condition, arrives at the stage capable of bearing sori a few days later than 
the general initiation of the crop. 
The gametangia arrive at maturity about the highest spring tide, and, 
as will be explained in detail in another section, they are liberated during 
a very short period of time soon afterwards. 
At the period of liberation, or a few tides after, the faint rudiments of 
the sori of the new crop begin to make their appearance. They are not 
localized on any definite region as are the tetrasporangia of Padina , but are 
scattered over the parts occupied by the preceding crop together with a 
short distal extension of it. With each successive crop the clear distal area 
becomes smaller and smaller, apical elongation becoming gradually slower, 
until at last the reproductive area reaches close to the apical cell itself. 
In a very short time after the escape of the gametes the walls of the 
gametangia degenerate and disappear, but, as already described 1 , the 
position of the liberated sorus is always indicated by the old basal cells, 
which are continuous with the cortex or limiting layer of the thallus, but 
rendered conspicuous by their increased size and different colour. At first 
they are much paler, owing to the small number and size of the chloro- 
plasts and the paucity of colouring matter in them. They gradually acquire 
a deeper colour, still the scars of the two last crops can always be dis- 
tinguished from each other. In the case of the male plants the raised 
borders of sterile cells is a further indication of the position of the old sori. 
These basal cells are incapable of giving rise to new gametangia, and when 
the whole or nearly the whole surface has been used up in this manner the 
plant dies, the process being hastened in some localities by the attacks of 
endo- and epi-phytes as well as of various animals. 
The following examples illustrate some of the above points a little 
more clearly : — 
i. A male plant 12 in. long and i in. wide below the first dichotomy. 
The distal l in. was quite clear of sori, and the current crop reached ijin. 
1 Ann. of Bot., xviii, 1904, p. 184. 
