142 THE RELATION OF PLANTS TO TIDE-LEVELS 
high-water mark. All these facts taken together show that for a plant growing, 
say, at the 4-foot level (7. ¢., near mean sea-level), the time for the most effective 
photosynthetic work, that is, the total duration per month of emergence during 
brightest daylight (9 a. m. to 3 p. m.) is reduced to about one-half that for a 
plant growing in the open, above high-tide level. In other words, while the 
exposure of an upland plant would be 6 hours in the middle of the day, that for 
plants at mean sea-level will vary from 0.25 to 6 hours per day, averaging 3 
hours per day for the month or season. Since the 4-foot level mentioned is 
about the average level of the photosynthetically active leaves of the lowest 
plants of Spartina glabra we have, in the 3 hours’ exposure mentioned, the 
approximate light minimum endured by this grass. Just what part this re- 
duced lighting may play in determining the lower limit of distribution is 
uncertain. Plants which are left submerged for a longer time than this by 
planting them at mean low water on the harbor bottom die out in one season. 
The effects of the various factors that are changed by this longer submergence 
can only be distinguished and determined by more prolonged experimental work 
than we have yet been able to carry out. 
The above given proportions, of light-reaching plants at different levels, were 
determined from the predicted tide-curves for Willet’s Point, New York, from 
May 1 to October 31, 1911, which are described on page 131 above. This curve 
for Willet’s Point is the closest approximation to that for Cold Spring Harbor 
that can be obtained, the chief difference in the two being the 0.4 foot greater 
height of high water, which would tend to slightly decrease the time of lighting 
of levels from mean sea-level upward. ‘Table D gives the times of exposure per 
day of four selected levels to total daylight, 1. e., from sunrise to sunset, and to 
brightest light, 7. e., from 9 a. m. to 3 p. m., for the month of May 1911, deter- 
mined from the above-mentioned predicted tide-curve. 
TABLE D.—Daily exposure of various tide-levels to daylight. 
Exposure to total daylight. Exposure between 9 a. m. and 8 p. m. 
Level. Hae aayoe ee Average ex- mit, 7 i Average ex- 
posure per |e 52 ee eee ee posure mer 
Minimum. | Maximum. day. Minimum. | Maximum. day. 
Hours. Hours. Hours. Hours. Hours. Hours. 
9 feet (and above).. 13.96 14.93 14.50 6.00 6.00 6.00 
i LOC Se cae scareelets 12.35 14.68 14.02 4.00 6.00 5.54 
Big6i LECT. 56,0: /Beisverauers 6 9.15 t 0.15 6.00 8.00 
O20 TO0tis fetes 0.0 5.60 1.5 0.0 1.25 0.12 
It is interesting to note here that the 0.0-foot level may not be exposed at all 
to daylight for 8 days at a time, and not be exposed at all between 9 a. m. and 
3 p. m. for 25 days out of the month. In this connection, however, we must 
recall again the often great clearness of the water in the center of the harbor at 
low tide, which allows plants at and below this level to get rather intense light © 
at low water, even though submerged by a foot or more of water. Since in the 
case of extreme spring tides high water always occurs in the middle of the day, 
the 9-foot level, if submerged at all, is covered between 9 a. m. and 3 p. m., the 
hours of brightest daylight. 
