358 DR. SARAH М. BAKER AND MISS М. H. BOHLING ON 
and nutrient salts are decreased, both by the fewer tides covering these levels 
and by the shorter period of immersion per tide. 
The conjunction of these two factors leads to a general dwarfing of the 
thallus, together with a decrease in spirality ; so that, in all the marshes, 
the uppermost levels are occupied by the minute turf-like forms of the 
e. ceespitosus type, and the lower levels by progressively broader, longer, and 
more twisted forms. 
Further, in the Ray marshes, along the creeks, that is at the lowest levels 
of distribution of the Fucus, the intermediate forms are always found at the 
mouths of the creeks, while the large, luxuriant specimens, with long, broad, 
much twisted thallus, are found in the creeks in the body of the marsh. 
This is probably because, at corresponding levels, the current is swifter along 
the narrow channels than at the mouth of the streams, and the Fuci have, 
therefore, a greater aecess of nutrient salts and dissolved gases during their 
short immersion by the tide. 
From these data, taken concurrently, one deduces that :— 
(a) Exposure to the atmosphere alone induces a shortening of the thallus. 
(b) Lack of nutrient salts, either through low salinity of water, short 
time in water, or sluggish current, induces a narrow thallus ; and, 
conversely, abundance of nutrient salts and dissolved gases makes 
for a broad, crinkled, and twisted thallus. 
These observations upon F. vesiculosus megecad limicola are confirmed 
by more general considerations. A reference to the table (p. 348) will show 
that the only marsh species which have a dwarfed habit are those derived 
from the lower levels of the rocky shore (see diagram of zonation on marsh 
and shore, fig. 14, р. 363). These Fucoids in the transition from rock to 
marsh have had their normal time of exposure by the tide increased, while 
their normal time of immersion has been decreased by the change. For 
this reason the change is associated with a dwarfed habit, which is not a 
necessary attribute of marsh Fucoids, as is shown by the normal size of 
Pelvetia canaliculata ecad libera, and also by occasional plants of marsh 
Fucus vesiculosus growing low down on the banks of deep streams, which 
may reach large dimensions (1 metre or more in length). 
The first general characteristic of the limicolous Fucoids—their dwarf 
habit—may therefore be referred primarily to two co-operating factors, — 
the lengthened exposure, which causes а shortening of the thallus, and the 
reduced immersion in sea-water, often accompanied by dilution, which causes 
a narrowing of the thallus. 
It is to be noted that the connexion between dwarf habit and long exposure 
is not limited to the limicolous Fucoids ; but, as is well known, the dwarf 
varieties of rock species, e.g. the var. nana of F. spiralis, the var. distichus 
of F. inflatus (see Borgesen, 1908, p. 720), or the var. minor of Ascophyllum, 
