III. Periodicity of the Sexual Cells in Dictyota. 535 
Comparison of Different Crops. 
If now we study the crop for late August and early September, 
1902, and compare the results, as represented in Diagram II, with those 
just described, we find that although there is general agreement between 
the two, there are several differences in matters of detail. We first 
observe that general liberation is delayed until about the fifth tide 
after the highest spring. Now as maturation and liberation are so evidently 
dependent on the spring tides, one would naturally expect the process 
to be accelerated directly as the height of the spring tides. Here, 
however, although all the spring tides in the second series are higher 
than those of the first, liberation, instead of being earlier, is actually later. 
This state of things occurred so frequently that it was at first conjectured 
that the increase in the amount of exposure during low water of the 
higher spring tides subjected the plants to more than their optimum 
of light or of heat, and in that way brought about their retardation. 
Though it is quite true that the plants are very sensitive to light, and 
possibly to variations of temperature, we now know that the above 
suggestion is not the correct one. 
There are other variations also which appear in the tabulated results 
of the observations. The length of the optimum line may be nineteen 
tides, as in the diagram quoted, or any number up to twenty-five tides. 
The width of the crop-band may vary from five tides to about ten, 
while the whole time occupied by a crop may range from twenty-four 
to thirty-four tides. 
In order to explain these variations it was natural to look for their 
causes in the fluctuations of the amounts of light and heat. Consequently 
careful records were kept of the state of the weather, especially as to 
amount of sunlight and the temperature of the sea. It is now found 
that the influence of these factors is quite secondary. Thus the liberation of 
the crop of September 23, 1899, took place five tides after the highest spring 
tide, the weather being very dull, cold, and stormy during the whole period. 
The crop of August 25 in the same year was liberated exactly at the same 
interval after the highest spring, though the weather during this period 
had been continuously bright and warm. Particulars relating to the 
weather are consequently omitted from the tables, and only such data 
are included as have been found to have an influence in deciding these 
variations. At the same time it must be remembered that extremes 
of illumination or of temperature may assist other causes in producing 
acceleration or retardation. 
When all attempts to explain these variations on meteorological 
grounds had thus proved abortive, the solution was sought for in the 
fluctuations of the tides themselves. Now, although the main fact of 
