TIDES AND EVAPORATION 137 
For the alge growing on the vertical stone walls and piles of the wharves, also, 
the upper limit of distribution is evidently determined by the amount of desicca- 
tion the plants can withstand. We have seen, e. g., (pp. 65 and 67) that 
Rhizoclonium and Fucus go highest in crevices in the wharf, or on the north 
sides of piles, where not reached by the sun and by winds. 
In like manner, for erect-growing seed plants between tide-marks, like 
Spartina glabra, the elevation attained is evidently conditioned, at least in part, 
by the desiccation it is subjected to. That is, by the length of time at each tide 
that its leaves are exposed to the dry air. A clear indication that this is the case 
is found in the fact that on certain shaded areas with moist soil on the western 
shore, this grass grows at a level much above its usual upper limit. It displaces 
here its sun-loving competitor Scirpus americanus, which as usual holds sway at 
these higher levels on adjoining dry and sunny portions of the shore. That a 
grass with rather thick-cuticled, rolling leaves should be en- 
dangered by the amount of transpiration it can be subjected to 
just above the 6.5-foot level, while its rhizomes and roots are em- 
bedded in a practically saturated soil, seems surprising at first 
thought. But it is to be remembered that the humidity of the air 
about the upper half of the plant, which includes most of the 
well-developed leaves, may be quite low. This is especially true 
of the south shore of the Spit, which is just where the upper 
boundary of the Spartina is strikingly definite. 
That attempt was made to determine the evaporating power 
of the air in this habitat of Spartina on the Spit, and in several 
other typical habitats by means of a porous-cup atmometer. For 
this purpose three atmometers were used, whose coefficients of 
correction had been determined by comparison with a Livingston 
standard atmometer. All the readings here given are corrected 
readings, and may therefore be directly compared with the Liv- 
ingston standard. 
For use at levels where, because of the danger of submergence 
by the tides, exposures can be made for only a few hours at a time, 
except occasionally during a series of neap-tides, an atmometer is = Fi. 5. 
required which will indicate very small losses of water. For this i tetera 
purpose the porous cup was attached to the shorter arm of a tige-marks. 
U-tube, the other arm of which was graduated to tenths of a 
centimeter by filling from a burette. The resulting instrument (fig. 5) is a 
simplified form of that described by Livingston 1906. Three instruments of 
this kind, which we will designate as Nos. 1, 2, and 3, were run simultaneously 
for a week in an instrument shelter, to discover possible leaks and to check up 
their relative rates. 
_ For three days in August 1909, during a series of neap tides, the three 
atmometers were exposed in different places and simultaneous readings made at 
the beginning and the end of each of the following periods: August 6, 11°00" 
a. m. to 12°40" p. m.; August 7, 7°20" a. m. to 12°20” p. m., and August 7, 
12°20” p. m. to August 8, 7°30" a.m. The total exposure for each instrument 
was 25.8 hours. The days on which the exposures were made were clear. The 
temperature recorded by a Friez thermograph in the shelter ranged from 18° 

