The Causes of the Zoning of Brown Seaweeds. 65 
The mean values are :— 
3 F.W. = 10 bundles expelled. S.W. = 4 bundles expelled. 
$ F.W. = 97 „ „ S.W. = 64 
The values in italics are those in which the number in fresh water 
is not greater than in sea-water. The majority of these experi¬ 
ments were made with only a short time of exposure, for the sake 
of convenience. 
A similar set of experiments with Fucus serratus gave mean 
values :— 
3 (means of 7 experiments) F.W.=31 bundles expelled. S.W. = 11 
? „ 2 F.W.= 113 „ „ S.W. = 51 
Mr. E. J. Salisbury has suggested to me that the concentra¬ 
tion of the film of sea-water covering the plants when they are 
exposed by the tides is an important factor in the extrusion of 
gametes. In view of the function of the paraphyses in this 
mechanism 1 think it is extremely probable that osmotic effects of 
this kind are utilized by the plant. At any rate the mechanism is 
by no means a simple one, and is probably made up of several 
factors ; among these the swelling of the paraphyses on any 
lowering of osmotic pressure may, 1 think, be reckoned. 
IV.— Check Experiments by Weighing the Gametes. 
As the methods employed throughout all these experiments 
are open to so many objections, it w T as thought advisable to repeat 
one series of experiments, using an entirely different method, as a 
check on the values obtained. These experiments were performed 
in the Laboratories at University College, London, on account 
of the apparatus required. The material was obtained from 
Mersea Island, Essex. The method employed was as follows. 
Fifty receptacles of Fucus spiralis were very carefully washed with 
filtered sea-water, and then dried in an oven for a definite time at 
20"C (as these experiments were carried out in November and 
December, and the others in August, the temperature of 20"C was 
chosen to imitate summer conditions). They were then put into 
filtered sea-water. After twenty hours the receptacles were removed 
one by one from the liquid, washing each with distilled water into 
the vessel. The remaining liquid was filtered through a tared 
asbestos gooch crucible, with the help of a suction pump. After 
thorough washing with distilled water, the crucible containing the 
oospores was dried at 55"—60"C for some hours, cooled in a des- 
