ENHALID FORMATION 21 
ment much smaller than this. For example, a series of young plants averaging 
26 mm. in length increased to an average length of 55 mm. in 20 days. The 
average daily rate of growth was 1.5 mm. and the maximum daily increment 
was 2 mm. 
It is of course possible that the large sheets of Ulva may winter over in some 
of the deeper, more protected parts of the Inner Harbor, and less probable 
that they may be washed in from the Outer Harbor. The rates of growth actually 
observed indicate that the largest of these plants can not be produced from 
spores in a single season. It is hoped that observations now under way may 
definitely determine the age of these larger sheets. It is of course possible 
that the larger amount of sewage present in the water in summer, during the 
session of the Biological Laboratory, may enable the Ulva to grow more 
rapidly than in winter or spring. (See Cotton, 1911, and Letts and Richards, 
1911.) 
The lower limit of distribution of attached plants of Ulva in the Inner 
Harbor is very near the mean low-water level. Plants which are drifting with 
their supports and torn off bits may be found at greater depths, but we have 
not determined how long they can persist there, and there is no adequate 
evidence that zoospores of Ulva develop to young plants at levels more than 6 
inches below mean low water. Why this species is confined to levels which are 
exposed to air and light has not been determined, but apparently it is due to 
the direct effect of some physical factor, since there are, in most places, no 
competitors, and there are, at least in some places, suitable substrata for attach- 
ment some distance below this limit. 
Since Ulva occurs in the next higher zone, we may discuss the factors 
determining its upper limit in that connection. 
Ulva probably influences the distribution of other plants only as it forms 
the substratum for certain epiphytic diatoms and a few species of blue-green 
alge, of which latter Spirulina is the only form of any importance. It is 
probable also that the smooth sheets of Ulva lying on the mud may prevent 
_ young seedlings and broken off bits of Zostera from getting a hold in the mud. 
Finally, the floating masses of Ulva, like other flood trash, may smother out 
many square yards of Spartina patens, or other plants already established on 
the marshes, by settling on them with the fall of the tide. The bare patches 
so formed often become re-covered first with Vaucheria or seedling Salicornias, 
as will be described later. 
Enteromorpha clathrata: This plant, as we shall find, is not by any means 
confined within the 1.5-foot contour line, but may be found as high as the 6 or 
6.5-foot level. It does, however, occur far more abundantly on the bottom of 
the harbor than elsewhere. As one rows over the harbor just before low water 
on a quiet day, he will see on the leaves of Zostera numerous tufts of sparsely 
branched, crinkled, green, tubular filaments, 1 or 2 mm. in diameter, and com- 
monly from 5 to 20 cm. in length. These are the young plants of probably a 
few weeks growth of Hnteromorpha clathrata. Observations made in the 
Outer Harbor show that this species may grow from the zoospore to a length 
of 4 inches in as many weeks. After these plants have grown to 8 or 10 inches 
many of them, like the young plants of Ulva, are broken away from their 
supports and float about in the harbor, to finally grow into the great tangles 
that settle down on the Ulva and Zostera, sometimes covering dozens or scores 
