856 DR. SARAH М. BAKER AND MISS М. Н. BOHLING ON 
THE CORRELATION BETWEEN THE HABIT OF THE vor AND THE PHYSICAL 
CONDITIONS UNDER WHICH THEY GROW. 
The above, then, are the main differences between rock and marsh, as far 
as they affect the algal vegetation. It remains to determine which of these 
physical factors is responsible for each of the general peculiarities of marsh 
Fucoids. The whole question is much simplified by the prevalence of vege- 
tative reproduction on the marsh. This does away with the possibilities of 
variations, due to the crossing of different strains, and ensures that all the 
observed variations are actually the direct outcome of a response to changes 
of environment. On the other hand, it сап be urged that the absence of the 
stimulus, obtained in the process of sexual reproduction, may cause profound 
modifications of a degenerative kind in the organism ; and this idea has been 
elaborated in a recent paper on the dwarf Fuci in the Baltic by Arcichovskij 
(1905). It is probable that some effect of this kind is produced by long- 
continued vegetative reproduction ; but the observations to be recorded seem 
to show that this is not the primary cause of the extraordinary changes of mor- 
phology correlated with the marsh habitat. In this investigation it has been 
found impracticable to make artificial cultures of the plants, chiefly because 
of the great labour involved in the provision of salt-water tides and currents, 
especially ina London laboratory. We have therefore relied entirely upon 
an ecological study of the question, 7. e. observations under field conditions. 
(a) Тнк Dwanr Hanrr. 
The most obvious morphological peculiarity of marsh Fucoids is their 
dwarf habit. The origin of this dwarfing has been chiefly studied by means 
of the varieties of F. vesiculosus; but the main principles observed seem 
equally applicable to the other species. 
Efect of Physical Factors upon the Morphology of F. vesiculosus 
megecad limicola. 
The simplest way of finding out the correlation between the physical 
environment and the morphology of a particular plant is to study intensively 
the distribution of the varieties of that plant in as limited an area as possible, 
This is the plan we have adopted in the case of the highly variable F. vesi- 
culosus megecad limicola. In the system of marshes fringing the estuary 
of the River Blackwater in Essex, the plant is abundant, and it shows there 
an amazing range of form. The chief marshes are shown in line shading in 
the sketch-map (Pl. 30) taken from the ordnance survey of the district. 
The soil of the marshes is very uniform, being composed everywhere of a 
rich black mud. 
