III. Periodicity of the Sexual Cells in Dictyota. 533 
beyond the scars of the last crop. Thus the elongation during the last 
fortnight was only one-third of that for the preceding fortnight. 
2. Female plant in its third crop. The proximal clear area was ij cm. 
long, that occupied by the scars of the first crop 3 cm., the second crop 
7 cm., and the current crop 13-5 cm., while the remaining 3 cm. at the 
distal end had not started reproduction. There is in most cases a pro- 
gressive increase in the extension of the reproductive area in the case of 
the first three or four periods. 
Vigorous plants produce such a number of sori that they soon exhaust 
themselves and decay. Many plants that start fruiting early are found in this 
condition by the end of September, and the first violent gale sweeps many of 
them away. The fact that we find many young plants in their first crop in 
early October proves that they must be the offspring of- tetrasporic plants 
of the current season. There is thus a continuous succession of young 
plants commencing the process of reproduction from the end of June to 
mid-October. 
The History of a single Crop of Sexual Cells. 
In order to understand the course of events, it will be convenient to 
follow in detail the development of a single crop from the first appearance 
of the rudiments to the discharge of the mature gametes. Diagram I shows 
the results of observations at the Swillies in the Menai Straits during the 
early part of August, 1901, plotted down in such a manner as to show the 
relation borne by the various stages in the development of the crop to the 
heights of the tides. The vertical columns represent the days of the month, 
while the curve in the upper part shows the hours of high water and the 
heights of the tides in feet. The lower part of the diagram is utilized to 
denote the stage of division of the antheridium as seen in surface view. 
Thus, a small cross placed on the lowermost line indicates an undivided 
rudiment, and a similar one on the sixth line shows that there are sixty- 
four cells in surface view. After the final division of the antheridium a 
short time elapses before maturation is completed and liberation sets in, 
and during this interval the only division that occurs in the oogonium 
rudiment — that which separates the stalk-cell — is accomplished. In the 
diagram this is indicated by means of the dotted line. Finally, the stage 
at which liberation of gametes takes place is denoted by inserting the 
marks above the tenth line. 
The first observations were made on the sixth of the month, when 
most of the antheridial rudiments were found to be undivided ; a few 
were two-celled and a very small number four-celled. On the eighth 
and tenth the plants gathered showed hardly any advance, but by 
the twelfth the great majority of the antheridia were in the two- or 
four-celled stage. By the fifteenth many were apparently mature, and 
