III. Periodicity of the Sexual Cells in Dictyota. 547 
It has already been explained that the plant never fruits in the sea 
before June, and also that a fruiting plant transferred into a culture vessel 
in the laboratory may complete and liberate the current crop, but it is 
very unusual indeed for it to mature another. In October of last year 
two plants of Dictyota , one male, the other female, were placed in a glass 
dish about eight inches in diameter and containing about an inch of water. 
The dish was then closed with a lid and left absolutely undisturbed ; the 
lid was not raised until the beginning of April in the following year. 
When examined the bottom of the dish was covered with what appeared 
to be brown dirt, but which on closer examination turned out to be 
a multitude of healthy little germlings, evidently the result of normal 
sexual fusion and not of parthenogenesis. The parent plants themselves 
had disappeared, with the exception of a number of little tips of branches, 
not more than J inch long. When these were examined every piece 
was found to bear young sori, and these were all in the same stage of 
development. Two days afterwards the oogonia were apparently mature, 
and in two days more they all were liberated, while on the succeeding 
day the oospores had completed their first segmentation. In the same 
manner two more crops were developed. The first and second took 
about the usual time to pass through the various stages, but the last of 
the three was a weak one : many of the gametangia failed to mature, 
and after that the little branch-tips died, partly from exhaustion, but 
partly, it is to be feared, as a result of the interference of the experimenter. 
Judging from the fate of numerous other cultures, the following 
was probably the course of events. The two plants when first transferred 
into the dish suddenly stopped the production of crops for a considerable 
period. After a time, however* they were able to adapt themselves to 
their changed environment, and when the necessary metabolic processes 
had been going on (very slowly, it is true), there came at last a time 
when the slow accumulation of reproductive energy caused the simultaneous 
initiation of sexual cells over the whole plants without the intervention 
of any kind of external stimulus. Gradually the older, basal portions 
became exhausted, died and decayed, leaving the bigger branches beyond 
the first dichotomy free. Still later, the smaller branches became separated 
in the same way, but each separate branch, however small, continued 
its rhythmic production of gametes, and on all the separate little pieces 
the process was carried on simultaneously . 
After labouring to demonstrate the complete dependence of the 
periodicity upon the tide-table, it is startling to come upon a case like the 
above, where there cannot possibly be any circumstance external to the plant 
to suggest any rhythm in reproduction. It may be argued that inasmuch 
as these individual plants had been subjected to tidal influence when 
growing in the sea, they may have acquired the periodicity in such 
