302 DR. SARAH M. BAKER AND MISS М. Н. BOHLING ON 
None of the species showing true marsh forms are devoid of traces of sexual 
reproduction, and it is to the distribution of the rare receptacles that we 
must look for evidence as to the factors which prevent the production 
of sexual organs on the marsh. 
The first fact which appeared was that the presence of halophytic vegeta- 
tion was distinctly prejudicial to the production of fruit-bodies on the 
associated Fucoids. Thus, both Fucus vesiculosus megecad limicola and 
Ascophyllum nodosum ecad scorpioides are frequently found in fruit during 
the spring and summer in stations, such as stream-banks or bare mud-flats, 
where halophytes have not yet colonized the ground ; but they hardly ever 
fruit when they are associated as undergrowth with phanerogamie vege- 
tation. This holds both in the Blackwater marshes and at Blakeney for 
the ecad volubilis of F. vesiculosus; and Mr. Cotton tells us that the fruiting 
specimens of the tiny ecad cæspitosus and ecad muscoides found at Clew Bay, 
Ireland, were also confined to stream banks or the periphery of the marsh, 
while the general undergrowth vegetation was sterile. 
А rather unexpected confirmation of these observations was the finding of 
three fruiting specimens of the free-growing Pelvetia by Prof. Oliver in 
January 1915. Pelvetia does not normally fruit to any extent in the winter 
months, and it seemed remarkable that such an unusual phenomenon as 
fruiting on the free-growing form should occur exactly at the season when 
fruiting is least common on the attached plant. The incongruity is explained 
by the reduction in the foliage of Aster and Salicornia, among which the 
Pelvetia was growing, during the winter months. 
There are two conceivable methods by which the presence of phanerogams 
might influence the aecompanying undergrowth, either by affording shade 
(or reducing СО.) and so decreasing its photosynthetic activity, or by con- 
tributing to the humidity of its surroundings during the periods of exposure 
and so preventing changes in the concentration of its cell-sap. The former 
hypothesis, that light was the determining factor, was especially promising 
in view of the observations of Lloyd Williams (1904 & 1905) on Dictyota 
dichotoma, which produces its sexual organs at regular intervals, governed 
by the good illumination prevailing at low spring tides. 
It happens, fortunately, that at Blakeney Point there is the means of 
testing these alternative hypotheses. The great Pelvetia-Salicornia marshes 
in that area are intersected by a shingle beach, crowned with sand-dunes, 
called the Long Hills (see sketch-map, fig. 1, in Baker, 1912). Along the 
lower edge of the north-western slope of this shingle-beach, there 1s a zone, 
about 2 feet wide, of normal Pelvetia canaliculata attached to pebbles, which 
are held rigid in a shallow layer of mud. The ground slopes towards the 
marsh and the shingle immediately below it affords good drainage, so that, 
in the intertidal periods, this zone becomes very much drier than the flats 
of the salt marsh, which are vertically only a few inches below it. These 
