556 Lloyd Williams . — Studies in the Dictyotaceae. 
12. A study of herbarium specimens shows that periodicity in the case 
of Dictyota dichotomci obtains in other seas besides our own, in localities 
widely separated from each other and extending as far as Australasia, and 
in several other species of the genus. There is as yet no evidence to show 
that it occurs in seas where there are no appreciable tides. 
13. In the case of the tetrasporic plants of all our British genera of 
Dictyotaceae there is no trace of periodicity. Throughout the season, and at 
every stage of the tides, sporangia of all ages may be found together 
on the thallus. 
14. In a laboratory experiment fragments of plants collected in 
October and left undisturbed in a small glass dish produced a succession 
of crops the following April. All their stages were quite similar, the length 
of the crop period was exactly the same as if the plants had been in the sea, 
and liberation and fertilization took place in a perfectly normal manner. 
The natural inference is that periodicity is a hereditary character. At the 
same time the preceding observations show with equal clearness that in 
seas where there are tides the periods of the crops are directly regulated 
by the alternation of high and low tides, so as to secure for the crops the 
maximum advantage possible from the increased illumination during low 
water of spring tides. 
1 5. The above observations apply to ‘ healthy ’ plants. Such plants 
require for their development : — 
(1) Ample light, without exposure to much direct sunlight at low 
water. 
(2) A continuous circulation of pure, clean water. 
(3) Freedom from endophytes, epiphytes, and dirt. 
Where these conditions are not secured the following results may 
ensue : — 
(1) Delay in commencing the reproductive season. 
(2) Retardation of the crop. 
(3) Inequality in the ages of the sori. 
(4) Reversion of half-developed gametangia to the vegetative con- 
dition. 
(5) Degeneration of more advanced gametangia. 
In quiet bays without much shelter the plants are apt to suffer from ex- 
cessive insolation, especially if the weather happens to be very bright during 
low water of equinoctial tides : this is generally associated with the attacks of 
parasites, which are particularly abundant in such waters. In such cases the 
antherozoids are discharged from the mature sori in a passive condition. 
The lower temperature and diminished light towards the close of the 
season result in smaller sori and a great increase in the number of aborted 
gametangia. 
16. A solitary specimen of Dictyota was collected whose sori had both 
