MARINE ALG OF BEAUFORT, N. C. 397 
the inner jetties at Fort Macon by means of oyster tongs and diving, a day being chosen 
when the water was about 15 cm. below the usual low tides. This showed alge occur- 
ring abundantly to a depth of about 75 cm. below the usual level, then becoming scarcer 
and ceasing about 1.4 m. below this level, none being found as low as 1.7m. In October, 
1906, one of the jetties at Fort Macon, being undermined by the current, sank to a depth 
of about 6 m. When the rocks of this jetty were dredged up the following July they 
were entirely bare of alge, although in the previous autumn they had borne numerous 
plants of Fucus, Sargassum, Dictyota, Hypnea, and other species occurring in this 
locality. 
The lower limit of the alge in this region is undoubtedly determined by the turbidity 
of the water and the consequent great diminution of the light penetrating to even 
moderate depths. It has been shown that the light reaching a depth of 90 cm. has an 
intensity of not more than 15 per cent of that of full sunlight, and that from 60 cm. to 
1.2 m. there is a great decrease in the strength of the light. It will be observed that it 
is just at these depths that the alge become scarcer and finally cease. 
This turbidity, however, besides affecting the amount of light, probably itself plays 
a part in limiting the depth to which the alge may grow, since these will receive sedi- 
ment from all the water above them, and so will receive more deposits the greater the 
depth of the water covering them. 
It is worthy of note that, while the alge in the harbor grow to a depth of only 1.4 m., 
those on the coral reef grow to a depth of 25.5 m. This is undoubtedly due to the 
greater clearness of the water over this reef. All of the plants of Brongniartella, Dasya, 
Grinnellia, and Nitophyllum gathered from this reef were exceedingly pale in color, 
being much paler than plants of Dasya and Grinnellia growing in the harbor at the same 
time or than plants of Brongniartella and Nitophyllum observed in summer. This pale 
color may have been due to the weak light occurring at that depth or to a combination 
of this and other factors, but we do not yet know enough about the color of alge to 
venture an explanation. 
Except in the spring, the upper limit of the great majority of algz in this region is 
determined by the height of the usual lowest low tides. Lyngbya confervoides, Hydro- 
coleum, several undeterminable species of Myxophycez, mats composed of minute plants 
of Enteromorpha, Ulva, Cheetomorpha, and Cladophora, and plants of Fucus, Gelidium, 
Gymnogongrus, and Actinococcus occur between tide lines; but, except for these 
species and occasional plants growing in shaded or otherwise especially favorable loca- 
tions, all alge occurring here in summer are strictly limited to the zone below low tide. 
This is undoubtedly due to the intense insolation and heat to which the exposed plants 
are subjected, the air temperature sometimes rising to 36°C. At the time of the spring 
tides, when the range of tide is greatest, low tide occurs here about noon, so that all the 
algee above low water are exposed to the sun during the hottest part of the day. Plants 
of Gracilaria, Hypnea, Chondria, Herposiphonia, and Nitophyllum have frequently been 
observed with a part or all of their thallus exposed by successive very low tides, and in 
every case they had been killed to the level of the water. Dictyota, Padina, and Rosen- 
vingea seemed slightly more resistant, since plants that had been similarly exposed 
appeared uninjured in some cases, but at other times they too were killed to the water 
level. While a single very low tide, caused hy the wind, may kill the exposed parts of 
the most tender species, it has little effect on the range of the alge, but the successive 
