130 THE RELATION OF PLANTS TO TIDE-LEVELS 
great a proportionate submergence. The alge on the mud among the Spartina, 
such as Fucus vesiculosus spiralis (plate xv1), Rhizoclonium, etc., apparently 
suffer also from the winter waves, aided by the ice. At least they are relatively 
scarce in the early spring. 
On the banks of tide channels, e. g., on the west side of the Inlet, or on the 
concave banks of the Creek (near 100 south) the turfs of grasses are often under- 
mined by the swift current and many pieces fall into the water to be carried 
away. Large numbers of plants and very considerable areas of soil are thus 
destroyed. 
Another way in which tidal currents and waves may injure plants is by 
covering them with tide-trash so deeply as to smother them out. Instances 
in which Spartina glabra, S. patens, Salicornia, and other species are thus 
destroyed over areas of several square meters are mentioned in Section ITI. 
Of the secondary effects of water-currents on submerged plants, the most 
important is probably the effect of this movement on the concentration, in the 
water about the plant, of solutions of useful and injurious gases or solids, and 
on the rate of interchange between the plant and the immediately surrounding 
water. It is a well-known fact that the hard bottoms of swift-flowing tide- 
creeks, or bottoms just below low-tide mark on wave-beaten shores, have an 
unusually luxuriant algal flora. While this is partly due to the stony bottom 
found under swiftly moving water, yet it is evidently attributable partly to the 
effect of the agitation of the water on the plants themselves. It is probable that 
all of the effects above mentioned are of importance. But no one, so far as we 
know, has yet proved experimentally whether the movement of the water is 
more important in simply increasing the rate of interchange of material between 
the plant and the surrounding water, or in bringing to the plant solutions of 
substances needed by it and removing waste substances cast off from it. The 
action of the waves in floating out the tangles of rockweed of the mid-littoral 
‘belt, and of then keeping them in almost constant motion, indicates that both a 
better aeration of the water and a higher rate of interchange of nutrient and 
waste substances between water and plant must result. These would be very 
interesting points to settle definitely by carefully checked physiological 
experiments. 
The influence of water-movement on a plant may also be exercised second- 
arily through the favorable or unfavorable effect of this movement on its 
competitors. For example, it is evident that the inability of large sheets of 
Ulva to withstand the strong current in the Inlet prevents the huge sheets of 
this species from covering large areas of the bottom here, as it does in the 
quieter parts of the Inner Harbor, and thus prevents it from smothering out 
many of the species that now find congenial conditions in the Inlet. On the 
other hand, the drifting by the tide of the rolls of Ulva mentioned on page 20, 
and the final settling of these on patches of Zostera and Ruppia, may smother 
these latter out in the same way that the masses of Ulva, Enteromorpha clath- 
rata, or of other tide-trash, have been shown to smother Spartina glabra, and the 
algal felts with it, in the mid-littoral belt. It is probable that it is the inability 
of other brackish-water alge to gain, or maintain, a foothold in such a swift 
current that gives Ilea fulvescens such undisputed sway on the steeper pebbly 
bottoms of the Creek between 150 south and 500 south. 
