SUBSTRATA 123 
of the diatoms Melosira and Navicula, of Enteromorpha clathrata, and still 
more frequent tufts of the red alge Ceramiuwm rubrum and C. strictum. A 
single leaf of Zostera may often bear two or three dozen tufts of these various 
alow, and the leaves are often broken off by the weight of this load, to be finally 
stranded on the beach. The two Ceramiums are practically confined to the 
Zostera. Aside from an occasional plant, on a pebble or shell in the Inlet, these 
Floridez find no other resting-place in this mud-bottomed harbor. In addition 
to these larger forms, Zostera may bear thousands of small, sedentary diatoms, 
like Cocconeis, and sometimes many square feet of a stand of Zostera may have 
the leaves fastened together and weighted down by the gelatinous colonies of a 
Spirulina. 
Ulva serves not merely for the attachment of Cocconeis and other diatoms 
occurring singly or in very small colonies, but may occasionally bear young 
plants of Enteromorpha clathrata and may also be weighted down by the gelat- 
inous colonies of Spartina just mentioned. 
Besides the three important species above mentioned that may serve as sub- 
strata for epiphytes, many seed plants of the upper littoral belt, such as Sali- 
cornia, Sueda, and especially Spartina patens, may, like Spartina glabra, have 
felts of Rhizoclonwwm and various blue-green alge tangled about their stems. 
Finally, any alga in the harbor, if of considerable size, may bear epiphytic 
diatoms of various species. 
B. NON-LIVING SUBSTRATA. 
By far the larger number of species found in the Inner Harbor, aside perhaps 
from the diatoms, grow on a non-living substratum of either purely inorganic or 
partly organic origin. These non-living substrata may be grouped as follows: 
(1) soils, including gravel, sand, mud, humus, and peat, among the constituent 
particles of which the holdfasts, 7. ¢., roots and rhizomes, of the seed plants are 
embedded; (2) solid substrata, including rock, stones, pebbles, shells, and 
wood. ‘To the surfaces of these the holdfasts of the various alge are attached 
without penetrating appreciably into their substance. Of course, it is evident 
that felt-forming alge like Lyngbya, Rhizoclonwum, etc., may grow on the 
surface of peat, sand, or gravel. But to these alge, since they do not penetrate 
these substrata, the latter are the equivalent of solid substrata. The pebble of 
the south shore of the Spit is to a Calothrix what a stone of the wharf is to a 
Fucus or Ascophyllum. 
1. Sorts AS SUBSTRATA (GRAVEL, SAND, Mup, HUMUS, OR PEAT). 
The soils about the harbor differ in fineness from fine silt or mud up to sand, 
or even pretty coarse gravel. They differ largely also in the proportion of 
organic content from nearly pure sand or gravel to humus and peat with a very 
large proportion of material of organic origin. 
_ There is a very distinct horizontal zonation evident in the general distribution 
of soils about the natural shores of the Inner Harbor from the bottom up to the 
10 or 12-foot level. As has been mentioned in speaking of the distribution of 
Zostera, the portions of the bottom lying below 1 foot are chiefly of a sandy, 
shelly, or pebbly character. The deep hole near 1,400 north by 600 east, and 
the deeper parts of the channel leading to the Outer Harbor, have a bottom of 
