Natural History of District of Columbia— McAtee 89 



The struggle for place must have been intense, however, for 

 conditions on the Piedmont were unfavorable. Probably in 

 many cases a strip back of the beach was the only suitable 

 habitat for the Pine Barren plants. During the last retro- 

 grade movement, however, these plants must have found 

 numerous favorable habitats, and where these have not been 

 destroyed by drainage, by covering with other deposits, or 

 by operations of man, the plants remain to this day. What 

 happened may be compared to the movement of plants dur- 

 ing the glacial periods. Their response to the progress of 

 the ice is undoubted, and during the retreat of the glaciers 

 bodies of northern plants were stranded here and there and 

 have persisted usually on mountain tops where the condi- 

 tions are most like those to which they were acclimated and 

 where competition consequently was least severe. 



So with the Pine Barren plants. The movement of this 

 flora toward, if not onto, the eastern part of the Piedmont 

 Plateau, when the latter was depressed, and their retreat 

 (before competing species) when it was again elevated must 

 be compared to the flow and ebb of the tide, which leaves 

 merely a bit of foam at one point, a little pool at another, 

 but, where conditions favor, a whole lagoon. What may be 

 thought of as persistent spume from this vegetational tide 

 are the few colonies of Pine Barren plants lodged in the 

 Appalachian Mountains. 64 Our Magnolia bogs represent the 



M See especially Kearney T, H. "The lower austral element in the 

 flora of the southern Appalachian region. A preliminary note." (Science, 

 N. S. 12, pp. 830-842, Nov. 30, 1900), and The Pine-Barren Flora in the 

 East Tennessee Mountains. (Plant World, 1, No. 3, Dec, 1897, pp. 33-35). 

 In the latter paper the following typical pine barren plants are mentioned : 

 Eupatorium album, Ascyrum stans, Juncus aristulatus, Pogonia divaricata, 

 and Itea vlrginica. The suggestion is made that "probably, • • * they 

 are the lingering survivors of a more southern flora, once widely dis- 

 tributed over the southern Apalachian region." (p. 35.) This theory is 

 approved by Norman Taylor (Flora of the Vicinity of New York, 1915, 

 pp. 25-26. The relict theory may be entertained, if we assume that the 

 whole region over which Pine Barren plants are now found was once 

 occupied by the Pine Barren flora, but that this has since been crowded by 

 an aggressive new flora out of all but the most favorable areas. But neces- 

 sary premises for this conception do not seem probable, since it is doubt- 

 ful whether the largely non-autophytic Pine Barren flora, which now 

 seems to have such strict limitations as to soil and other edaphic condi- 

 tions, ever was predominant in the soils resulting from simple disintegra- 

 tion of the metamorphic rocks. At any rate the relict hypothesis will not 

 explain the presence of Pine Barren plants in our Magnolia Bogs and 



