92 
PACIFIC SCIENCE, Vol. Ill, January, 1949 
salinity is doubtless the limiting factor. This 
would be mitigated in direct relation to the 
amount of rain that falls on the island and to 
the distance that salt water has to diffuse 
through the porous limestone and debris making 
up the island. Mesophytic conditions would 
exist in direct proportion to the predominance 
of outward flow of fresh water over the inward 
diffusion of salt. The pattern of the distribu- 
tion of vegetation in relation to the general 
rainfall and to the land area of an islet is exactly 
what would be expected on this basis; the more 
mesophytic types of vegetation exist in the 
interiors of larger islands in relatively rainy 
regions. Native agriculture is a reflection of 
this also. Coconut and breadfruit culture is 
more or less coincidental with the inner, more 
mesophytic, forest, and the excavated taro 
patches are found in the interior, least brackish 
locations and are absent on the driest islands. 
On very dry islands there is no agriculture. 
The distribution of herbaceous seed plants, 
general wherever there is sufficient sunlight, is 
a reflection of the leaching of the salt from the 
upper layers. Ferns are found only in the shade 
of the mesophytic forests. 
The few mangroves found on atolls, again, 
reflect the same pattern. Bruguiera conjugata 
and Lumnitzera coccinea, which are found on 
high islands toward the inner edges of the 
mangrove swamps where the salinity is less, 
are on atolls in the brackish mangrove depres- 
sions, while the Rhizophora and Sonneratia, 
which grow on the seaward sides of the swamps, 
are found on atolls on the highly saline shores 
of lagoons and in swamps with direct tidal 
channels. 
Further support for this principle is found 
in the flora and vegetation of elevated atolls 
and reefs. Here, even with a small degree of 
elevation, the flora shows a marked increase in 
number of species and vegetation becomes more 
mesophytic. This is demonstrated in the slightly 
raised portion of Anaa Atoll, the more strongly 
raised Henderson Island, the high platform of 
Makatea Island, as well as in the jungle-covered 
limestone plateaus and cliffs of the Marianas 
and the southern Palaus. The salt would nat- 
urally be largely leached out in these situations. 
Summary: The atoll flora, both native and 
introduced, is restricted to such species as can 
tolerate considerable salinity. The vegetation 
is controlled by the salinity of the ground water. 
The fresh water of an atoll islet exists in the 
form of a very shallow Ghyben-Herzberg lens, 
with its freshness directly proportional to the 
product of the rainfall and the distance from 
the beaches. The vegetation, as a result, is meso- 
phytic in relation to the same factors, tending 
to be most luxuriant and arborescent toward the 
center of large islets in wet regions, and more 
sparse, desert-like, and predominantly herba- 
ceous or dwarf-shrubby on beaches, spits, and 
small islets in climatically dry areas of the 
ocean. 
