THE BROWN SEAWEEDS OF THE SALT MARSH. 351 
A. Effect of Changes in Salinity of the Water on the Fucus. 
Travelling up the River Blackwater, from Mersea Island to Northey 
Island, the salinity of the water in the channel decreases considerably. 
The amount of this change must vary greatly with the state of the tide and 
the drainage of rain-water into the river-basin ; but we had not the oppor- 
tunity of collecting a series of samples at different stations to obtain 
comprehensive data. One series of salt-water samples was, however, taken, 
on April 12th, 1914, upon a moderate spring tide in fair weather, which 
may be taken as representing tolerably average conditions. 
The concentration of chlorides, calculated as sodium chloride, at four 
stations at high-tide in mid-channel, is given below; at the first three 
stations this value was calculated by taking the mean of two observations, 
one taken on the rising and the other on the falling tide :— 
Opposite Ray Island ............ 2:85 per cent. NaCl. 
Opposite Tollesbury ............ 2:82 per cent. NaCl. 
Opposite Osea Island ............ 2°75 per cent. NaCl. 
Opposite Northey Island ...... 2:58 per cent. NaCl. 
On this day there was about 10 per cent. decrease in the concentration of 
salt, at the island furthest up the river ; but no doubt after rain, or at the 
neap tides, a very much greater dilution may occur. 
The effect of this dilution on the morphology of the marsh Fucus vesiculosus 
is very striking. In the various marshes lining the river, from the mouth to 
Northey Island, there is a gradual and progressive attenuation in the thallus 
of the Fucus, until at Northey Island, the longest specimens have the appear- 
ance figured (fig. 15, p. 366) of a slightly flattened skeleton Fucus showing 
moderate spirality. This attenuation was shown, but to a less extent, in the 
smaller forms. 
In this connexion, it is interesting to notice that Techet (1908), experi- 
menting on several genera of brown, red, and green marine algæ, none of 
them, however, closely allied to Fucus, showed, by means of detailed cultures, 
that in general a decrease in salinity of the water caused the alge to assume 
a form more slender and less branched than under normal conditions. 
B. feet of Exposure to the Atmosphere. 
The different levels of any one marsh are covered by a different number of 
high tides—a drop of a few inches in level being sufficient to allow access to 
one or more extra tides in each fortnightly cycle. This means that the higher 
levels suffer from two disadvantages: («) the exposure at the neap tides is 
prolonged by several days; (b) the opportunity for the absorption of water 
LINN. JOURN.—BOTANY, VOL. XLII. 20€ 
