202 The Causes of the Zoning of Brown Seaweeds. 
The specimens of Fucns vesiculosus grew rapidly in jar A, and 
none of the plants died. In jar B this seaweed grew quickly, but 
some of the specimens became rather shrivelled towards the 
end of the time. In jar C only two specimens just survived, 
grew very little and shrivelled at the base. 
Ascophyllum nodosum grew well in jars A and B, and one 
specimen survived in C, though much stunted. 
Fucus ceranoides was very slow growing in all the jars, but it 
grew best in B, where there was exposure for half the period. 
None of the specimens died. 
From the way in which the seaweeds grew, one would draw 
the following conclusions :— 
1. That the species of seaweeds growing high up on the shore 
have a power of resisting dessication, which is not possessed by 
those growing lower down, and that this power decreases regularly 
in those species growing towards the lower levels. 
2. That the seaweeds which can best resist dessication grow 
most slowly, and those that grow most quickly are the least 
tolerant of dessication. 
The species which resist dessication most must have most 
protection from loss of water, and hence will absorb water most 
slowly; and since these plants get their nourishment from general 
absorption of water by the thallus, the best protected plants must 
grow most slowly. In the case of Fucus ceranoides there seems to 
be a definite physiological adaptation against dessication, for it grows 
best when it is dry for a large proportion of the time. It is probable 
that, growing on the rocks, the seaweeds would never become so 
dry as in the glass jars, which were left exposed to all the weather 
conditions; but the results obtained give an indication of the 
reasons for the zoning observed. 
In the lower zones the primary factor is probably rate of 
growth; the quickest growing forms would supersede the others. 
In the upper zones the determining factor is tolerance of dessica¬ 
tion. It is easy to see that these factors would cause fairly sharp 
zoning of the forms with these varying properties. There may 
be also other factors depending on the reproductive systems of 
the adult plants on the dispersal of the gametes, and on the power 
of germination of the zygotes under different conditions. 
I have to thank my brothers for kind assistance in carrying 
out this work. 
