54 
Sarah M. Baker. 
ON THE CAUSES OF THE ZONING OF BROWN 
SEAWEEDS ON THE SEASHORE. 
II. —The Effect of Periodic Exposure on the Expulsion 
of Gametes and on the Germination of the Oospore, 
By Sarah M. Baker 
(Un iversity College , LondonJ. 
S was stated in a previous paper on this subject (New Phyto- 
logist, Vol. VIII., p. 196, 1909), it seems probable that the 
most important physical factor governing the zoning of Brown 
Seaweeds is the periodic exposure by the tides. In order to find 
out, as far as possible, why some species grow high on the shore 
and others lower down, it is necessary to work out in detail the 
effect of exposure on the different species. Some experiments on 
the effect of periodic dessication on vegetative growth have already 
been described ( loe. cit.) ; the work described in the present paper 
was carried out with a view to determining the influence of this 
factor on the germination, and also on the dehiscence mechanism 
of the four species under observation. Some details are added on 
the mode of expulsion of the gametes from the conceptacles. 
Except where otherwise stated, all these experiments were 
carried out at White Cliff Bay, Bembridge, Isle of Wight, during 
July and August; and were made, as far as possible, on the 
seashore. 
The order of zoning observed on the shore was as follows: — 
(1) Fucus spiralis , L. ( =F. platycarpus, Thur.) (2) Ascopliyllum 
nodosum. (3) Fucus vesiculosus. (4) Fucus serratus ; beginning 
from the high water line and going downwards. 
1.—Effect of Periodic Dessication on Germination. 
To obtain ripe gametes, receptacles of both kinds were left dry 
for a few hours, and then put into filtered sea-water. The water 
containing the eggs and sperms was mixed and left standing twelve 
hours to ensure fertilisation ; it was then divided equally between 
three straight open glass tubes, the lower ends of which were 
covered with filter paper protected by muslin on each side. The 
tubes were provided with wire hooks by means of which they could 
be hung inside glass jars of filtered sea-water, and so be filled with 
