128 THE RELATION OF PLANTS TO TIDE-LEVELS 
The stone of the wharves about the harbor is chiefly a brown sandstone. There 
are, however, numbers of large blocks of granite and gneiss scattered among the 
brownstones of the wharf on the east side, from 1,000 north to 1,600 north. 
Certain yellowish blocks of this granite and gneiss are constantly bare of 
Ascophyllum and Fucus, though all the surrounding stones of otherwise similar 
character and the brownstone blocks are densely covered by these alge (plate 
111). As no differences could be discovered in the chemical or physical charac- 
ters of the barren and the alga-covered rocks (see p. 70), we have no explanation 
to suggest for the striking difference in their alga population. In speaking of 
the gravelly soils above the 6.5-foot level, we have mentioned that Calothriz, 
Lyngbya, or Microcoleus are attached to the surfaces of the pebbles of the upper 
littoral beach, forming “ Phycochromaceta ” of Warming (1909, p.175). These 
simple forms are attached to the surfaces of these fine pebbles and sand grains 
just as the larger alge of lower levels grow on the larger pebbles of the Inlet, or 
of the channels of fresh-water rivulets along the shore (plate x). 
The wooden channel-stakes of the middle of the harbor form, as was noted 
just above, a restricted but often densely populated substratum for numerous 
alge. Thus a single stake may bear, attached to its bark, or, in older stakes, to 
the bared wood, groups of tufts of Melosira, Navicula, Ulva, Enteromorpha 
clathrata, Ralfsia, Dasya, Grinnellia, and Porphyra, besides felts or tangles of 
Rhizoclonium mingled with various blue-green alge. On the larger piles and 
wharf-logs and on bits of heavy wreckage along the shore, the algal population 
may be much richer in both individuals and species. Thus, e. g., the vertical 
chestnut piles of the wharf of the Research Laboratory may bear a dense drapery 
of rockweeds, any gaps in which are largely occupied by felts of Lyngbyas and 
Rhizocloniums, by warty incrustations of Ralfsia, by an occasional Porphyra, 
or by dense colonies of Bostrychia. In the winter Ulothrix flacca becomes 
prominent on these same piles. Near high-water level occur bands of felted 
' Lyngbyas and tufts of Calothriz. All of these alge, except the finer-felted ones, 
are attached to the firmer parts of the wood, and careful study of sections of the 
holdfasts of Ascophyllum and Fucus, of Bostrychia and Porphyra, show that 
these do not really penetrate into the tissue of the wood, but simply spread over 
the surface and into the furrows between the harder strands of the wood. The 
wooden wharf-logs are generally too near high-water level to bear much rock- 
weed, but they often have an abundant felt or tangle of Rhizocloniwm even at 
the 8-foot level when on the north side or where the log is kept moist. At the 
8-foot level on logs and stones of the Research Wharf grew the only species of 
lichen found near high-water mark. This lichen, Lecanora subfusca, occurs 
also at this same level on stakes on the Marsh. 
2. THE INFLUENCE OF WATER-CURRENTS. 
Under this head are included the effects of water-movement in streams, tidal 
currents, and waves. Such water-currents may affect the distribution of plants 
directly, as by wafting about the plankton of the surface and the drifting plants 
of the bottom, or by the dispersal of the spores or seeds when shed. They may 
also cause injury or even the total destruction of plants on shores or wharves by 
carrying ice against them or dropping flood-trash upon them. On the other 
hand, these currents may affect plants secondarily, by determining either the 
character of the substratum, the different degrees of aeration of the water over 
different areas, and finally by a favorable or unfavorable effect on competitors. 
