20 THE RELATION OF PLANTS TO TIDE-LEVELS 
original attachment in the Inlet and on other pebbly bottoms of the harbor. 
Such detached plants of Ulva do not necessarily lodge within the Ulva zone 
the first time that they are washed into the harbor; neither do they stay 
indefinitely at the point where they first lodge. Even larger sheets that have 
been resting on the bottom for days or weeks may be moved about by the water 
in one of two ways. In the first place, sheets that lie on the flats bordering the 
tide-channels may be rolled up by the tidal current of large, swiftly flowing 
spring tides, and gather additional sheets as they are tumbled along over the 
flats, until rolls are formed 0.5 meter in diameter and 2 or 3 meters long. 
These rolls have been seen to roll for 40 or 50 meters over the shoals near 
2,000 north by 200 to 800 east. Such rolls may evidently either be carried out 
to the Outer Harbor, or be broken up again, with considerable tearing of the 
sheets, and the fragments floated back to be redistributed over the bottom of 
the harbor. The long bare strips of bottom noted above are often merely the 
trails of such rolls. 
The second mode of transportation is one that is seen on days that are bright 
and windy, during extreme low water, especially of spring tides. ‘The long 
exposure of the Ulva-covered parts of the bottom at such times allows the 
water to drain off and permits air, and probably other gases from the underlying 
mud, to collect under the coarsely crumpled sheets. With the rising of the 
tide these sheets or portions of sheets of Ulva are floated up 2 or 3 feet off the 
bottom or even to the surface at high water, and finally, becoming entirely 
free from the bottom, they are blown by the wind along the surface of the water. 
Such floating plants may drift about the harbor and then out through the 
Inlet with the next fall of the tide, or they may settle in new places on the 
bottom, or with stronger winds they are sometimes blown to the shore, where 
they may become tangled among the reed grass or drifted on the beach. 
Windrows of Ulva thus formed are often found on the beaches between the 
3 and 8 foot levels, or caught in the Spartina glabra, sometimes covering many 
square meters. There the plants finally die from exposure to the sun and rain. 
Just what part this floating of the Ulva may play in finally denuding the 
bottom of the harbor was not determined by an actual counting of the floating 
sheets and a measurement of their sizes, but it is evident that it may be a very 
considerable one. It is, however, probably small in comparison with the 
destruction and transportation of Ulva accomplished by the ice, though the 
importance of this factor is also undetermined, since we have not been able to 
study the harbor in detail in winter. 
If the history of the sheets of free Ulva found on the harbor bottom in July 
is that which has just been suggested, then it is certain that the growth of 
those plants which settle in favorable places must be very rapid. One of the 
larger sheets measured in August was found to be 10 meters long and about 
equally broad. The production of such a plant as this in 3 or 4 months from 
one 6 or 8 inches across indicates the remarkable average rate of radial growth | 
at all parts of the margin to be 30 or 40 mm. per day. 
Assuming that the largest plant seen in July 1911, located just aside from 
the swiftest current, had come from the largest plants seen in April 1911, the 
rate of growth would average only 8 or 10 mm. per day. Actual measurements 
by Miss Stella G. Streeter of the rate of growth of somewhat smaller plants 
growing under natural conditions in July and August 1910 give a daily incre- 
