The Causes of the Zoning of Brown Seaweeds. 67 
V. — Conclusions. 
We are now in a position to estimate, to a certain extent, 
some of the effects of dessication on these four species of algas. 
The results have all been obtained under more or less artificial 
conditions; but they may serve to indicate some of the causes of 
the zoning observed on the shore. These effects may be briefly 
summarised. 
Those seaweeds which grow in the upper zones are capable of 
resisting dessication both during germination and vegetative 
growth. Also their receptacles are protected by being filled with 
mucilage, and this seems to make their dehiscence mechanism 
most efficient when they are dry for a considerable length of time. 
Also, in both Fucus spiralis and Ascophyllum nodosum the para- 
physes project considerably from the ostioles of the conceptacles; 
and this may be an adaptation to ensure the gametes being very 
rapidly expelled during the short time they are covered by water. 
In a few experiments with Fucus spiralis it was found that during 
the first hour, after putting a dry receptacle into water, half the 
number of bundles of gametes were given off that were expelled 
during twenty-four hours. 
‘ The seaweeds growing in the lower zones have become adapted 
to very rapid growth ; they are thus able to supersede the more 
slow-growing and protected forms, in their own zones, but at the 
same time they have not the power of resisting dessication, so that 
they cannot grow in the upper zones. Also their dehiscence 
mechanism has become efficient for very short times of exposure ; 
and they are able to choke out any stray member of a higher zone, 
growing lower down, which cannot compete with them in the 
number of reproductive bodies given off. At the same time in 
Fucus serratus, the lowest form, the receptacles are very little 
protected. 
On the whole it seems as though the greatest competition had 
been called into play in the lowest zones, the dry and uncongenial 
regions of the upper shore being left to the most tolerant forms, 
which, if left to themselves, are able to grow anywhere on the 
shore. 
I wish to express my warmest thanks to Professor F. W. 
Oliver for his kind interest and encouragement; also to Mr. A. D. 
Cotton for his advice and assistance, and to Miss M. K. Bohling 
for assistance in some of the experiments. 
