546 
Lloyd Williams. — Sltt dies in the Dictyotaceae. 
Is Periodicity an Hereditary Character? 
The question which naturally arises at this point is — to what extent 
is the phenomenon of periodicity in Dictyota due to heredity ? There 
are two possibilities which may be considered. We may conceive in 
the first place, of sensitiveness to a light stimulus being transmitted to 
the sexual generation in such a way that in all cases where there is 
a rhythmic fluctuation of light there will also be a corresponding periodicity 
in the production of gametes. Thus the tendency would always be present, 
only requiring the appropriate stimulus to manifest itself. In localities 
where there are no perceptible tides this stimulus would be absent, and 
consequently there would be no periodicity of crops. In the alternative 
case we may conceive of the character being so firmly fixed upon the 
sexual generation, that periodicity would manifest itself under any 
conditions, whether there would be any corresponding fluctuation of 
light or not. Should this supposition be the correct one, then the role 
of the light is merely to control the periodicity by fixing the periods 
of initiation and liberation, but not in any way to originate it. 
At first the facts seemed to point to an ideal case of direct induction 
of a character by environmental conditions during the ontogeny of the 
plant. * Thus the crops of June and early July appeared very vague and 
indistinct in their periodicity, for on the same plant would be found 
sori of several different ages. One might naturally suppose this to be 
due to the plants being as yet too young to have acquired the rhythm. 
Following this came the more and more distinctly marked periodicity 
of August and September, succeeded in October and November by 
retardation and irregularity, as the conditions became altered. The whole 
hypothesis seemed to fit the facts in a most admirable manner, but it 
was soon shown to be illusory. If only the portions of the July plants 
already mature be considered, the crops are found to be perfectly normal 
in their time of development, and in the case of the October crops the 
retardation Really follows the changed light arrangements, and the finding 
of old and young stages together late in the season is chiefly due to 
the failure of many of the sori to become quite mature. Thus the 
periodicity is perfectly regular throughout, and there are no evidences 
of its being induced. 
It would be very interesting to ascertain what happens in those 
seas where there are no appreciable tides ; still more instructive would 
it be to cultivate the sexual plants from tetraspores. This is a most 
difficult undertaking, but it is to be hoped that some means will be 
found of getting plants under culture to thrive as in the sea. The 
history of one of the experiments tried by me is very curious and is 
exceedingly important in this connexion. 
