WATER-CURRENTS 129 
The plankton organisms, such as the Diatomacee and Peridinee, are often 
drifted together in certain corners of the harbor by tidal currents and winds till 
they color the water deeply, while other parts of the harbor are comparatively 
free from these plants. The distribution of many Chlorophycee and Floridee 
that are free or attached to small supports is changing constantly and they drift 
with the tide. The ultimate results of this drifting is often the stranding of 
these plants so high up on the bottom or shore of the harbor that they are 
killed by exposure. We have mentioned in detail in Section III (pp. 18, 21), 
the repeated redistribution of plants or fragments of Ulva and E'nteromorpha 
clathrata, and the same process must be very active in the case of Fucus vesi- 
culosus spiralts in late winter and early spring, when the wearing off of the dead 
stalks of Spartina by ice and waves has left the Fucus free. 
It is evident that spores or seeds discharged into moving water may be carried 
to very considerable distances by it. The seeds or spores of plants, e. g., those 
living in the tidal channels, must thus be distributed widely over most or all of 
the habitats suitable for them, as well as over many others. We suggested above 
(p. 32) that certain Floridex, of sporadic occurrence in the Inner Harbor, 
probably arise from spores which are brought in by the tide from neighboring 
areas in the Outer Harbor, where they are present year after year. A very 
interesting question that arises here, which can be answered by experiment only, 
is whether these spores can become attached to surfaces of stone, shell, or wood 
while the tidal currents are still running, or whether it is only for a short time 
at slack-water that an effective holdfast can be developed. The cases of alge 
like [lea and Monostroma, in streams near high-water level, are of especial 
interest in this connection, since the only period of slack-water in these habitats 
is that at high water, and then the pebbles on which the spores are to start are 
surrounded by salt water. The fresh water of the streams at this time, as was 
shown by Miss Streeter, runs out on the surface of the harbor, leaving salt water 
next the bottom.* It may prove true that the spores of these plants, which are to 
live most of the time in fresh water, do, and perhaps can, germinate only in salt 
water. If this is the case it would offer an interesting explanation of the fact 
that these alge do not spread up the streams beyond the high-water mark. 
In winter, when ice is abundant in the harbor, the plants of the shore, and 
especially the alge on the piles and walls of the wharves, are subjected to pretty 
serious grinding by the cakes of ice and may even be frozen into the ice during 
very cold weather and then torn off. Tufts of Spartina glabra, a meter square, 
were found in July 1910, many yards away from the nearest Spartina. In 
1910, e. g., the small tuft at 2,200 north by 750 west was a newcomer. The 
only plausible explanation we can offer of the appearance of this tuft in an area 
which in 1909 was totally bare of Spartina is that ice froze about the stalks of 
the plant at high tide, in early winter, and that later, with a higher tide, the 
clump of the grass, with the peat-mass on which it grew, was lifted bodily and 
floated by the ice to its present position. In this case the grass has persisted for 
two (or three) seasons. In other cases, where the turf is dropped in the middle 
of the harbor, 1. e., in deeper water, it does not flourish, probably because of too 
8 Our tests of the Creek at high water of an 8-foot tide showed the presence, just 
above the bottom, of a stratum of water about 1 foot thick with a density of 1.020 
which extended upstream to 600 south. A layer about 2 feet in thickness next above 
this had a density varying from 1.020 to 1.014. The water at the surface upstream 
from 450 south is entirely fresh, while from this point northward it increases in 
salinity to a density of 1.010 at 50 north. 
