ENHALID FORMATION 19 
On closer examination of the Ulva plants of the harbor, it is found that 
practically the only attached plants present are relatively small, being 1 to 5 
or 6 dm. across. These are found chiefly on pebbles and shells along the sides 
of the channel of the Inlet leading to the Outer Harbor and beside the main 
fresh-water streams, e. g., at 200 north by 550 east, at 200 north by 950 east, 
at 2,380 north by 980 west. A few attached plants were found on stakes, on 
buoys, and on shells of living mussels in various parts of the harbor. A very 
few attached plants occur on the rhizomes of Spartina glabra and on stones 
of the wharves in the next higher belt. The widespread, though sparse, dis- 
tribution of these plants in the harbor probably indicates that zoospores are 
abundant there. The practical absence of attached plants from most parts of 
the harbor bottom is evidently due to the lack of proper substrata, except where 
entering streams or tidal currents leave a surface of coarse particles on the 
bottom. ‘The detached sheets of Ulva covering the bottom may become second- 
arily fixed by the attachment to them of numerous mussels or of snails. In 
other cases parts of the sheet of Ulva may be buried, and thus fixed, by the mud 
shifted by water-currents or by burrowing animals. 
The general distribution of Ulva described above is that found each summer, 
in July and August, for six years past. The exact size and position of the 
minor bare spots in the Ulva zone changes from year to year, and in fact from 
week to week or even from day to day, due to the movement of the free Ulva 
by water-currents. In December 1912 the Ulva was about as abundant in the 
harbor as in summer. The bottom of the harbor was reported as nearly bare of 
Ulva in February 1913. In the following April, however, Dr. A. F. Blakeslee 
found many small sheets on the mud at the south end of the harbor. 
A somewhat detailed examination of the harbor in April 1911 showed that 
the bottom above mean low water is much less completely covered with detached 
Ulva than in midsummer. In fact, large sheets were almost wanting in those 
parts of the harbor that could be examined. On the other hand, a search for 
attached Ulva, in the places where it occurs in summer, showed that it was far 
more abundant in April. On the east side of the channel to the Outer Harbor, 
for example, there were thousands of attached plants of Ulva, of all sizes up 
to 1.5 or rarely 2 dm. across, and all of them were evidently growing vigorously. 
In July 1911 these same areas bore a much smaller number of plants, of which 
the largest in the middle of the channel were about 6 or 8 dm. long. In the 
inrushing tide near the Inlet one may always find plants of Ulva or parts of 
plants floating in with the current, and thus being carried to the quiet parts 
of the Inner Harbor, many of them settling on the bottom with the next fall 
of the tide. These floating sheets are evidently plants that have been broken 
off from their supports or substrata in the Inlet and elsewhere where young 
plants are developed abundantly. The maximum size of fixed plants found in 
the Inlet in July 1911 (about 6 dm.), is probably the size at which the tensile 
strength of the base of the plant is just able to withstand the strain of the 
swiftest tidal currents. Plants larger than this are torn off, or carried away 
with their supports, as even smaller ones also may be, and are thus added to 
the covering of the bottom of the Inner Harbor. 
It seems clear from the facts just mentioned that the large, free sheets of 
Ulva in the Inner Harbor are developed by the continued growth and crump- 
ling of the relatively small and flat plants that have been torn from their 
