26 THE RELATION OF PLANTS TO TIDE-LEVELS 
Ruppia maritima: This comparatively diminutive and delicate species is the 
only seed plant besides Zostera that is found on the bottom of the Inner Harbor 
below 1.5 feet. The characteristic mature plant of Ruppia, as it occurs in 
this harbor, is about 25 cm. long, and of 10 to 12 internodes varying in length 
from 20 to 70 mm. It has 8 to 10 functional leaves each 20 to 60 mm. long 
and 1 mm. wide. The plants flower freely at Cold Spring Harbor during July 
and August, but the stalk of the infloresences is short, rarely more than a few 
em. long, and therefore the flowers are not at the level of the water-surface 
except for about half an hour at the rise of the tide and a lke interval 
at its fall, 7. e., when the water-level is between 6 inches and 1.5 feet above 
mean low water. The plants flower and apparently fruit freely, as seedlings 
are rather frequently found. 
While Zostera is characteristic of an area lying below mean low water, 
Ruppra is practically confined to a vertically narrow belt between mean low 
water and 1.25 feet. The extreme limits are —0.5 foot and +1.5 feet. Though 
the vertical distribution of Ruppia is thus very limited, its horizontal distribu- 
tion is quite wide. At its lower limit, near mean low water, Ruppia is found 
mixed with scattered Zostera, but it is also scattered abundantly over areas 
near the 1.5-foot level, where Zostera is entirely wanting, e. g., 1,300 to 1,600 
north along the east shore. Nowhere does the stand of Ruppia become as dense 
as the denser stands of Zostera, and, in fact, areas where the bottom is actually 
covered by Ruppia are small and rare. This is due not only to the relatively 
small number of plants, but also to their delicacy. 
The areas where Ruppia is most abundant are those with a soft bottom, 
bare of Ulva and usually protected from currents and waves. Such areas are 
those indicated on plate x11. They are found chiefly in the western and 
northern parts of the harbor, but there is another such habitat with abundant 
Ruppia along the eastern shore behind the Zostera belt. The horizontal distri- 
- bution of Ruppia is, in other words, limited to areas of quiet water and the fine, 
muddy soil formed in such areas. The vertical distribution of Ruppia, on the 
other hand, seems clearly determined by tide-levels. The lower limit of this 
species, as we have seen, is mean low water, or a few inches lower in exceptional 
cases, and the upper limit is at 1 foot, or more rarely at 1.5 feet, above mean low 
water. That is, the plant never occurs where constantly submerged, but rather 
on areas which are exposed for from 0 to 4 hours each day, depending on the 
magnitude of the tide. It seems clear that competition with other species 
can not be an important factor in keeping Ruppia out of soils at lower levels, 
since Ruppia does not occur on bottom below mean low water that is bare of 
Zostera and Ulva. The character of this bottom is apparently identical with 
that on which Ruppia is growing a foot or two higher up. There seems to be no 
difference in the conditions at these two levels, except in the relative duration 
of submergence and exposure. 
The upper limit of distribution of Ruppia may be determined in part perhaps 
by competitors, e. g., Spartina glabra, in the shade of which Ruppia occasionally 
grows at its upper level. It seems more probable that Ruppva does not flourish 
above the 1-foot or 1.5-foot level because unable to withstand the exposure to 
desiccation by air and sun at low water. This view is supported by the fact 
that Ruppia is most abundant in areas where it is kept wet by little rivulets 
that run over the mud at low tide, e. g., along the edge of the tide-stream at 
