384 BOTANICAL GAZETTE [JUNE 
the greatest spring tides. The effect of aeration, temperature, pres- 
sure, and light is considered. The first three are deemed negligible — 
because plants in different situations subjected to different conditions 
in these respects show}the same behavior in fruiting. Light, then, 
is the remaining condition which, with the alternation of spring and 
neap tides, varies most for the plants of all situations, and this is 
believed to be the determining factor for the following reasons. (1) 
At Bangor the development of each crop is rapid at first, when there 
is comparatively little water over the plants at low tide and much 
light is obtained. The development is retarded with the approach 
of neap tides when the light is weak, and is accelerated by the follow- 
-ing spring tides when the available light again becomes strong (chart 
1). (2) Whether liberation shall occur on the third or fifth tide 
after the greatest spring tide depends on whether the preceding 
spring tides have been great or small, thus giving more favorable or 
less favorable light conditions for the rapid development of the early 
stages. (3) The length of time taken by any crop to pass from initia- 
tion to liberation is directly proportional to the number of days inter- 
vening between one set of spring tides and the next; in other words, 
to the number of days when the available light is weak. (4) During 
October and November the light becomes less intense at the low water 
of spring tides, and both initiation and liberation are delayed several 
tides. (5) At Plymouth, England, the crops seem to be produced 
wholly within one set of spring tides, and the time of liberation seems 
to be determined by the height of the tides during development. 
The light is less advantageous at the low water of spring tides, and 
liberation does not occur until seven to twelve tides after the greatest 
spring. 
These facts seem to show a causal relation between the amount of 
available light and the time of fruiting, but a single experiment tends 
to make this conclusion less certain. Two sexual plants of Dictyota 
were brought into the laboratory in October and left undisturbed 
during the winter. When examined the following April, they produced 
three crops at fortnightly periods, although they had been removed 
from the alternating influence of the tides for six months. WILLIAMS 
therefore concludes that “ periodicity of the sexual cells is an hereditary 
character and consequently may be expected to manifest itself in 
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