140 THE RELATION OF PLANTS TO TIDE-LEVELS 
C. EFFECT OF TIDAL CHANGES ON AERATION. 
It is pretty certain that for thick-cuticled plants like the Spartina, Distichlis, 
Salicornia, Sueda, etc., the period of submergence at high tide is one during 
which there can be little interchange of gases between these plants and the sur- 
rounding medium. At this time the stomata of the shoot are closed by water 
and, during the whole time of submergence, the only supply of O and CO, 
available for the plant is probably that held in the air-canals of the shoot. In 
the case of Spartina glabra this stored supply occupies a considerable portion 
of the bulk of the shoot. The whole system of subterranean rhizomes and 
roots of these plants is dependent chiefly on the shoot for its supply of gases, 
since the heavy, fine-grained mud in which the rhizomes and roots, e. g., of 
Spartina glabra are embedded is practically impenetrable to gases, except 
along burrows of the fiddler crab (Gelastmus pugilator) or those of the muskrat 
(Fiber zbethicus). Of course, except at spring tides, the upper portions of the 
leaves of the higher plants of Spartina glabra will still be exposed at high 
water, and it is possible that, by means of the abundant air-canals, gases may 
be exchanged between all parts of the plant and the outside air. 
For the other species mentioned above, since they grow higher up on the shore, 
the air-supply is not cut off for so considerable a time as for Spartina glabra, 
though this deprivation must still be of some consequence, since such a plant, 
e. g., as Salicormia, will, on the average, be more or less completely submerged 
for from one-fifth to one-third the daylight hours of an average summer day. 
The effect of submergence on the physiological activities of such plants could 
probably best be determined by growing them in a tide-pond the level of which 
could be maintained at a constant level for any time desired. 
For seed plants like Zostera and Ruppia, whose upper limit is not far above 
mean low water, it is probable that the necessary gaseous interchange is accom- 
_ plished during submergence, through the thin-walled epidermal cells. The 
same thing is perhaps largely true for the alge growing between tide-marks. 
Most of these, e. g., the rockweeds, Rhizoclonium, and the Schizophycee, have 
more or less gelatinous cell-walls, which quickly dry on exposure and so form a 
nearly impermeable membrane over the surface. On many days, it is true, only 
the outer exposed branches of Fucus or Ascophyllum and perhaps of other alge 
become really dry on the surface. Hence there may be a considerable gaseous 
interchange occurring even during low tide, a point which can be certainly 
determined only by experiment. 
D. EFFECT OF TIDAL CHANGES ON SALINITY OF SOIL-WATER AT 
HIGHER LEVELS. 
One of the effects of tidal changes may be that of periodically increasing the 
salinity of the soil-water. Near the 8-foot level, e. g., are certain areas where, 
during neap tides, the soil is barely moistened by slowly seeping fresh water, but 
at the fortnightly spring tides the salinity of the soil-water of these areas is 
increased by the salt brought up by the high tides. The effect in these cases is _ 
probably not large. In the case of the fresh-water tributaries of the harbor, 
the change in salt-content with tides of varying height is probably much more 
important. For example, there are growing in the larger streams, between the 
6-foot and 8-foot levels, alge such as Ilea and Hnteromorpha, which may be 
