342 



SCIENCE. 



[X. S. Vol. XVIII. Xo. 454. 



[Senecio halsamitw Muhl.] and Erigeron 

 Pers. [(L.) Pers.] (Barren Treeless Forma- 

 tion. Cera-stium Associatioii). 



F. East Side Grum Creek along Preston 



EUN. 



A large part of this exposure is treeless, and 

 upon the broken-down serpentine rock grow 

 mats of Phlox subulata L. {Phlox Associa- 

 tion), Trifolium agrarium L. [T. aureum 

 Poll.] Pteris aquilina L. \_Pteridium aquil- 

 inum (L.) Karst.], Verbascum hlattaria L., 

 Panicum latifolium L., Potentilla canadensis 

 L. and Cerastium oilongifolium Torr. (Cer- 

 asiium Association). The trees are the same 

 as the botanist finds on the other serpentine 

 barrens mentioned. Thickets of green briars 

 are also characteristic of the treeless areas 

 here. 



A study of the flora of these rocky exposures 

 reveals the fact that the same association of 

 species is not found on all of the serpentine 

 barrens. The several component species differ 

 as the localities differ, although the same gen- 

 eral character of the vegetation is preserved 

 by the presence of several dominant plants, 

 found on all of the barrens. The red cedar 

 Juniperus virginiana L., the barren oak, 

 Quercus nigra L. \_Q. marylandica Mueneh.] 

 the white oak, Quercus alba L., the sour gum, 

 Nyssa sylvatica Marsh., the sassafras, Sassa- 

 fras officinale ISTees [S. sassafras (L.) Karst.], 

 the smooth upland sumac, Rhus glabra L., the 

 red maple, Acer rubrum L., may be said to be 

 the dominant character species, while on most 

 of the barrens, although not found on all, 

 occurs the chestnut, Gastanea sativa Mill, 

 var. americana Gray [C dentata (Marsh.) 

 Borkh.]. When the growth of these trees is 

 dense the serpentine areas are rendered im- 

 penetrable in many places by the green briars, 

 Smilax rotundifolia L., Smilax glauca Walt., 

 the lianes, Vitis cestivalis Michx. and the 

 Virginia creeper, Ampelopsis quinquefolia 

 Michx. [Parthenocissus quinquefolia (L) 

 Planch], which festoon the trees and inter- 

 twine with each other to form a dark gloomy 

 forest inhabited by the cotton-tail rabbit. 

 Where the ground is too barren to support 

 trees, which usually grow in situations where 



there is considerable surface soil, the green 

 briar, Smilax rotundifolia L. associated 

 with Smilax glauca Walt, covers the ground 

 with a dense growth separated by intervals of 

 grass, where the botanist finds the small sun- 

 drops, (Enothera fruticosa L. [Kneiffia fruti- 

 cosa (L.) Eaimann], tufted hair grass, Des- 

 champsia cwspitosa Beauv., associated with 

 the blackberry, Bubus villosus Ait? (Gray) 

 [R. nigrobaccus Bailey], and meadow-sweet, 

 Spirwa salicifolia L. These treeless areas can 

 be distinguished at a distance by the clumps 

 of briars, by the presence of sentinel-like red 

 cedars, and by an occasional sour-gum tree. 

 The one herb found on all of the serpentine 

 exposures is the barren chickweed, Cerastium 

 oblongifolium Torr. [C. arvense L. var. oh- 

 longifolium Holl & Britt.], which varies from 

 a dwarf cespitose herb to one that, taller and 

 more distinctly branched, covers acres of 

 ground. Some of the barrens are distin- 

 guished by the presence of matted growths 

 of the moss pink. Phlox suhulata L. Such 

 are the barrens at Pink Hill (H) and along 

 Preston Eun (F), where extensive areas are 

 covered by this herb associated with the bar- 

 ren chickweed and the wooly blue violet, Viola 

 ovata Nutt. Upon one or two of the barrens, 

 viz., Westtown, Pa. (G), and Edgmont, Pa.,* 

 grows the fame fiower, Talinum teretifolium 

 Pursh. This plant is clearly controlled in its 

 distribution by edaphic conditions, for it is 

 found, and its nearly related species, Talinum 

 rugospermum Holzinger, on a variety of rock 

 formations throughout the eastern United 

 States.f The barren at the Williamson 

 School is noted for a growth of laurel, Kalmia 

 latifolia L., dwarf willow, Salix tristis Ait., 

 and until recently was visited by botanists 

 for the scarlet painted-cup, Castilleia coccinea 

 Spreng [(L.) Spreng]. 



* On the authority of Mr. Benjamin H. Smith, 

 who ascertained the locality from Mr. Witmcr 

 Stone. 



t Harshberger, J. W., ' An Ecological Study of 

 the Genus Talinum,' Bulletin Torrey Bot. Club, 

 XXIV., p. 182. 



Holzinger, J. M., 'The Geographical Distribu- 

 tion of the Teretifolium Group of Talinum,' A. 

 Gray Bulletin, VIII., p. 3G. 





