SECTIONAL TRANSACTIONS.—K. 455 
by the tide. When they are covered by the tide every day the mortality 
of seedlings is high. If they have a ‘threshold period’ free from tides 
the mortality of the seedlings is low. Density of individuals depends, 
therefore, upon frequency of submergence. 
Within a uniform ‘tide environment’ the individuals are distributed 
approximately atrandom. ‘There is a slight tendency to aggregation, which 
it is possible to estimate statistically. There is no obvious reason why the 
individuals of such a population should be aggregated. 
Dr. W. E. Isaac.—The distribution and zonation of marine alge on the 
coasts of South Africa (11.0). 
West coast of South Africa subject to influence of antarctic Benguela 
current. Sea temperatures fall northwards to Luderitz, then rise. East 
and south coasts (to Agulhas) directly under influence of tropical Mozam- 
bique current. Sea temperatures of region between Agulhas and Cape 
Point only partly affected by warm current ; marked seasonal temperature 
fluctuations in this region due to summer south-east winds. 
Algological investigations conducted at various points from Lamberts 
Bay to East London. 
Different temperature conditions have marked effect on species distribu- 
tion and algal ecology. Colder waters chiefly characterised by presence of 
extensive sub-littoral zone of large Laminariacee—Ecklonia buccinalis, 
Laminaria pallida, Macrocystis pirifera. Champia lumbricalis, essentially 
a cold water species, where it characterises lowest littoral zone. In warmer 
waters, Gelidium pristioides dominates a mid-tidal zone. Species of Cauler- 
pacee prominent in regions directly influenced by Mozambique current 
but absent from colder waters. ‘Throughout region investigated, Porphyra 
capensis the dominant species in highest littoral zone. This zone poorly 
developed in warmer waters, but dense, extensive, and with large-sized 
individuals in cold waters. 
Common features of littoral area in regions investigated: (1) Pheo- 
phycea not prominent; (2) Rhodophycee prominent as dominant and 
ah species ; (3) Porphyra capensis persistent as uppermost littoral 
alga. 
Mr. N. WoopuHeaD.—Algal periodicity in an Anglesey lake (11.30). 
The algal periodicity in Llyn Maelog, Anglesey, has been studied over a 
period of two years, plankton samples collected daily being available. 
Several species of Myxophycee form a succession of water-blooms early in 
the summer ; the dominant organisms have changed within the last twelve 
years. These are succeeded by a Tribonema phase occupying the position 
often taken by Myxophycean maxima in other lakes. The winter and spring 
months have a diatom plankton, rich in species. Of them, only Asterionella 
becomes abundant. It shows two maxima, a major one of longer duration 
in December and a secondary one in May. Between May and the rise of 
the Myxophycee is interpolated a short but well-marked Dinobryon phase 
as the temperature increases. Daily sampling reveals that the Myxo- 
phycee, during their period of maximum development, show great and rapid 
fluctuations in quantity. This appears to occur simultaneously in different 
parts of the lake. Over a hundred algal species have been added to the 
recorded flora of the lake. 
AFTERNOON. 
Cruise on Ranworth Broad (2.0). 
