-HARPER: VEGETATION OF THE HEMPSTEAD PLAINS 271 
fast as the older ones are killed by fire. The pines and oaks 
scattered over the eastern part of the Plains are of species not 
very sensitive to fire, so that they do not need to grow in dense 
groves for protection, like the prairie groves of the Mississippi 
valley.* The original boundary between prairie and forest here 
has been almost entirely obliterated by cultivation, but it was 
probably rather sharp in most places, for the regular forest trees 
of Nassau County, both north and south of the Plains, are mostly 
of species not very tolerant of fire, and the fires probably stopped 
f 
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ПРИЗА Е «с. m өзе» 
Fic. 3. Portion of pine grove ("Island of Trees") in prairie about a mile 
southwest of Central Park, showing especially Pinus rigida and Baptisia tinctoria. 
2:47 p.m., Aug. 25, 1909. In the absence of shrubby undergrowth this place differs 
notably from the pine-barrens of Long Island and New Jersey, and resembles some 
of those in the southeastern states. (For another view of the same grove see Bull. 
Am. Geog. Soc. 43: 359 
abruptly at the edge of the forest, where the shade kept the humus 
too damp to burn readily. 
It seems rather strange for the ground to be covered with 
lichens and sprinkled with mosses in an area subject to ground 
fires, for these plants are supposed to be very sensitive to fire; 
but probably any one spot on the uplands does not get burned 
over more than once in two years, on the average. And the 
commonest lichen is so minute and close to the ground that fire 
* See Gleason, Bot. Gaz. 53: 38-49. 1912; Torreya I3: 173-181. 10913. 
