368 DR. SARAH М. BAKER AND MISS М. Н. BOHLING ON 
condition of life, and, on the other hand, those which have the narrowest 
thallus are the oldest."  Arcichovskij (1905, part ii.) also emphasises the 
effect of phyllonecrosis in producing dwarf forms, coupling with it, as sub- 
sidiary causes, the dilution of sea-water and its contamination with organic 
debris. This effect, however, is clearly shown not to apply to Fucus vesi- 
culosus in general, from the specimen figured (fig. 16) in which the 
vegetative offspring or “older shoot" of a narrow form is a very broad 
form. The *exposure" of a Fucus may curtail its nutritive possibilities in 
another way, by reducing the time of immersion in water charged with 
СО», always supposing that photosynthesis in a marine alga can only take 
place under water, a supposition for which the experimental evidence has not 
Fig, 16.—Fucus vesiculosus megecad limicola ecad volubilis. Nat. size. 
Plant showing vegetative production of a broad form from a narrow form, 
yet been obtained, but which seems probable. If so, the dwarfing of these 
immersed forms takes place in a similar way, by curtailing photosynthesis ; 
first, by the reduced light intensity, at the depth where they grow, and, 
secondly, by the short supplies of CO,, due to the quietness of the water. 
. The rigid absence of sexual reproduction in the loose-lying Fucus resiculosus 
is precisely what one would expect in the uniform concentration of the tide- 
less and currentless sea-water in which they grow; in the same way, there 
are no serious concentration differences over the upper and lower surfaces 
of the thallus to induce spiral coiling or twisting. Hence the plant is 
symmetrieally branched in a radial fashion and shows no spirality. 
