TIDES AND LIGHT SUPPLY 141 
surrounded by pure fresh water continuously for 10 days and then, during 5 or 
6 days of spring tides, be subjected to strongly saline water for from 1 to 2 
hours at each high tide. As was suggested earlier, the effect of this submergence 
is probably not great, except on soils that are comparatively dry in the intervals 
between series of spring tides, since in wet soils the salt water occasionally flood- 
ing them probably does not penetrate far. 
On dry shores, however, there is a strip of soil between the 7.5 and 8.5 foot 
levels where the salt-content is probably increased during each series of neap 
tides. This may be brought about by the constant movement upward of the 
water in the soil by capillarity to levels above that of the high water of the neap 
tides. By the evaporation of the water from the soil the salt will continue to 
accumulate at the levels mentioned until the soil is flushed out by the high 
waters of the spring tides, or by rains. 
E, EFFECT OF TIDAL CHANGES IN EXPOSING PLANTS TO RAIN. 
On the effect of tidal changes in exposing plants to rain there are but few 
observations to record. It is evident that all shore and bottom plants above 
—1 foot may be subjected to a pretty thorough washing with fresh water by any 
heavy rain of the growing season that occurs during low tide. Many of these 
plants, like Spartina and the rockweeds, may be drenched with rain for 6 or 8 
hours at a time and not suffer from it. We have noted above that Fucus and 
Ascophyllum may lie for several hours in pure fresh water at low tide. In the 
case of certain of the red seaweeds, however, such as the Ceramiums on the 
Zostera, and plants of Agardhiella, Chondria, and Polysiphonia, a drenching of 
this sort, especially if followed by exposure to a hot sun, may cause the death of 
the plant. Large portions of the great sheets of Ulva are frequently found dead 
after exposure to such conditions. 
In general, all observations thus far made seem to show that nearly all the 
plants found above mean low water may withstand a more or less protracted 
wetting with fresh water, though only a few like lea, Hnteromorpha intes- 
_ tinalis, and Ectocarpus do, as we have seen in Section III, actually live where 
subjected to this every day. 
F. EFFECT OF TIDAL CHANGES ON THE LIGHT-REACHING PLANTS. 
It is evident that even in clear water the effective solar energy reaching sub- 
merged plants or parts of plants is markedly lessened by each foot of water it 
must pass through. In water of the turbidity of that often found along the 
shores of our harbor it is probable that submergence of a plant in 2 feet or, some- 
times, even in 1 foot of water will practically put a stop to photosynthetic 
activity. In the middle of the harbor the water is usually less turbid. In fact, 
on real quiet days it may be very clear at and near low water. 
From what has just been said it is evident that plants growing below high- 
water mark must do most of their photosynthesis during low tide. A reference 
to plate xxiv will show that since, on half the days of each month, high water 
occurs near the middle of the day (1. e., between 9 a. m. and 3 p. m.), it is evident 
that the most effective sunlight is, on these days, cut off from plants below high- 
water level for a longer or shorter time. This would be most markedly true of 
plants nearer low-water mark, but still true in some degree of all plants below 
