III. Periodicity of the Sexual Cells in Dictyota. 539 
tides for initiation and the earlier stages. The above shows this idea to be 
erroneous. The earliest rudiments are generally formed while the libera- 
tion of the preceding crop is still going on ; hence there is a slight but 
distinct overlap. The optimum of initiation takes place generally from 
four to two tides before the lowest neap. It seems then that as soon as 
the old crop has ceased its demands upon the plant, the ‘reproductive 
energy ’ becomes directed to the formation of new sori ; these make their 
appearance immediately. At first their progress is fairly rapid, but with 
the gradual lowering of the tides at neap, development becomes somewhat 
slower, to be once more greatly accelerated as the spring tides come on. 
Hence, if the optimum line were drawn with absolute accuracy, it would 
show a slight curve towards the vertical at each end, while the middle 
portion would be very nearly horizontal. This cannot be well shown in the 
diagrams, as the data are not sufficiently full. 
6. We have taken the divisions in the antheridium as indications of 
the rate of development of the crop. As already explained, there is only 
one division in the oogonium rudiment — the one which separates the stalk- 
cell. In the case of the tetrasporangium this division takes place at a very 
early stage ; here it is delayed until a few tides before the highest. 
7. In most of the cases which are fully recorded there is a fairly close 
and significant correspondence between the semilunation and the number 
of tides occupied by the whole crop from the very earliest initiation to the 
final clearance. In Tables I — 1 1 1 this is seen when columns 3 and 8 are 
compared for the crops numbered 4, 7, 8, 10, 12, and 16. Thus, although 
any given sorus takes much less than a semilunation for its development, 
the whole crop occupies practically the whole of the interval from a given 
tide to the corresponding one in the next series. 
8. Initiation and liberation during the earlier crops each occupies about 
ten to twelve tides. Later, in September and October, there is far greater 
uniformity as to stages of development, and the width of the crop-band 
is reduced to from six to eight tides, sometimes even fewer. This can be 
well seen by comparing Diagrams I and II. The principal reason for this 
is probably the cessation of growth, for elongation, as has already been 
shown, exercises a slightly disturbing influence on the regularity of the 
process. It is possible also that the recurrence of the crop-periods im- 
presses the rhythm more distinctly upon the individual ; this can be con- 
ceived of as possible whether we regard the periodicity as inherited or 
merely induced during ontogeny. 
9. Reverting to a point already mentioned, the existence on the distal 
portion of an elongating plant of sori very much younger than on the 
remainder of the thallus naturally suggests an inquiry as to their sub- 
sequent fate. Do they mature and liberate with the current crop, or 
remain until the succeeding one? Very careful observation of a large 
