136 THE RELATION OF PLANTS TO TIDE-LEVELS 
TABLE B.—Number of submergences per calendar month and for the season, from 
May 1 to October 31, 1911. 
{In each column, under the name of the month (or the season), the figures give the number of times that 
the level in question is submerged during that month or during the season.] 
Lével May. June. July. Aug. Sept. Oct. Season. 
* | (60 tides.) (58 tides.) (60 tides.) (60 tides.) (58 tides.) (59 tides.) | (355 tides.) 
Feet Times. Times. Times. Times. Times. Times. Times. 
6.00 60 58 60 60 55 58 351 
6.25 60 58 60 56 52 57 343 
6.50 60 58 60 54 52 53 337 
6.75 59 58 54 51 43 46 3ll 
7.00 52 54 52 44 43 39 284 
7.25 39 39 44 37 87 33 229 
7.50 33 33 38 34 So 30 200 
7.75 26 24 28 28 26 23 165 
8.00 23 14 21 22 21 20 121 
8.25 11 7 6 12 15 14 65 
8.50 8 5 3 2 5 ff 30 
8.75 7 4 0 0 3 4 18 
9.00 5 0 0 0 1 4 10 

TABLE C.—Number of exposures per calendar month and for the season, from May 1 
to October 31, 1911, of levels from—1.25 feet to 1.75 feet. 

May. June. July. Aug. Sept. Oct. Season. 
(60 tides.) (58 tides.) (60 tides.) (60 tides.) (58 tides.) (59 tides.) | (355 tides.) 
Times. Times. Times. Times. Times. Times. 
1 0 0 0 3 9 
3 2 0 3 4 17 
9 6 14 17 17 79 
26 27 28 28 27 165 
48 52 43 40 36 264 
58 60 55 48 46 323 
58 60 59 53 53 343 
58 60 60 56 55 348 
58 60 60 58 59 355 

.B. EFFECT OF TIDAL CHANGES IN WATER-LEVEL ON EVAPORATION OR 
TRANSPIRATION FROM THE PLANT. 
It is, in the first place, clear that when delicate, thin-cuticled plants like 
Zostera or Ulva are exposed at low tide during a warm, sunny day, they may be 
subjected to a dangerous desiccation. This desiccation, to which plants grow- 
ing above mean low water are liable, is undoubtedly concerned with determining 
the upper limit of distribution of such species. 
As has been mentioned in Section III, many plants of Cladophora, Entero- 
morpha, and Ulva, of various red seaweeds, and the leaves of Zostera, are often 
killed off by drying out on hot days in summer. The Ulva is oftenest destroyed 
by being floated to the higher levels of the beach by the air bubbles that collect 
under it on a hot day. 
We have spoken also (p. 91) of the drying out, and cracking to polygonal, 
tile-like blocks, of the felts of Schizophyceew and Chlorophycee occurring on 
the south shore of the Spit, between the 6.5 and 7.5 foot levels. At slightly 
higher levels these felts are usually wanting, probably chiefly because they and 
the soil bearing them are less frequently wet by the high tides, but are exposed 
to desiccation for a longer time. That the relative dryness of these higher levels 
really determines the absence of these algal felts seems clearly indicated by the 
fact that these felts may occur above their usual level when on the north or 
shady side of tufts of grasses, e. g., at 7.5 to 8 feet, on the top of the stone pier 
on the eastern shore. 
