TIDAL CHANGES Fel 
3. THE CHARACTER OF TIDAL CHANGES AND THEIR INFLUENCE ON 
PLANT DISTRIBUTION. 
In the earlier sections we have repeatedly referred to various effects of the 
tidal changes in water-level on individual species of plants. We may now look 
more closely into the magnitude of the tidal changes in water-level and the ways 
in which the latter may affect the growth, and other physiological activities, of 
littoral plants, and thus aid in determining their distribution. These peculiari- 
ties and effects of the semi-daily rise and fall of the tide may be discussed under 
the following heads: (A) Characters of the Tides; (B) Effect of Tidal changes 
on Evaporation; (C) Effect on Aeration; (D) Effect on Salinity of Soil-Water 
at High Levels; (EK) Effect on Exposure to Rain; (F) Effect on Light Supply. 
In addition to the effects just enumerated the tides cause part of the water- 
currents that are referred to above and also have an influence on the salinity and 
the temperature of the water of the harbor in general, which are to be discussed 
below. 
A. CHARACTER AND MAGNITUDE OF THE TIDES. 
The predicted, semi-diurnal range in water-level, due to tides, varied during 
the growing season of 1911 from 4.2 feet to 10.8 feet. (See plate xxiv, and 
Tittman, 1910.) The “mean range” during this season was 7.63 feet. This 
mean range varies from year to year, and the one here given for the growing 
season of 1911 is about 0.1 foot below the “ corrected mean range” for three or 
four decades. 
The smaller or “ neap range” occurs just after the first and third quarters 
of the moon in each month. This neap range varied, during the months from 
May to October 1911, from 4.2 feet on October 1 to 7.0 feet on June 20. The 
greater or “ spring range” occurs just after the new moon and the full moon. 
This varied in 1911 from 8.5 feet on July 14 (and 8.6 feet on June 10) to 10.8 
feet on May 27. (It was 10.7 feet on October 10 and even 11 feet on April 30, 
the day before the beginning of our somewhat arbitrarily fixed growing season.) 
These various facts are indicated graphically in plate xx1tv. The chart there 
shown was constructed from data given in the U.S. Tide Tables above referred 
to. (See also tables A, B, and C, pp. 135, 136.) 
Of course, the high water actually occurring at Cold Spring Harbor may 
sometimes be higher than that predicted, because of a northerly wind blowing 
the water into this long, narrow harbor. Or on another day high water may be 
lowered by a southerly wind retarding the inward flow of the water at flood-tide. 
On the other hand, the height of a low tide may be lowered: by a strong 
southerly wind or kept above the predicted height by the retarding effect of a 
northerly wind during ebb tide. In the long run, however, this influence of the 
wind in modifying the water-level at high and at low tide would prove about 
equal in both directions, and so the actual tides would show an average or mean 
range corresponding closely with that of the predicted tides. It must be 
remembered that any effect of a general prevalence of winds from one direction, 
é. g., from the southwest in summer, is one of the factors included in the actual 
tides observed at Cold Spring Harbor in 1894, on which observations the 
prediction of tides for this station is based. 
The general effect of the semi-diurnal variation of water-level, with high and 
low tides, on the vegetation of the shores, is probably dependent (see p. 14 
