670 Harshberger ; Plant formations of Nockamixon Rocks 



Nockamixon Rocks, Pennsylvania. Its preservation on the Nock- 

 amixon cliffs is probably due to two factors, viz., the coohiess of 

 the habitat under the rocks at the head of the glen with a northern 

 exposure and the isolation or inaccessibility of the shelves upon 

 which the plant grows. 



ES : 1 ne veg 



of the smaller rock shelves varies (figure 4). One shelf is occu- 

 pied by Cerastinm arvense var. oblongifolium, another by Gcranhnn 

 Robcrtianum, another is fringed with a grass, MuJilenbcrgia tfrna- 

 flora {M, Willdetumii). Still other shelves are characterized by 

 Poa dcbilis, Oryzopsis vtclanocarpa (0. racemosa), Anemone virgin- 

 iana, Potentilla canadensis, Henchera aniericana, Saxifiaga virgin- 

 icnsis, Galmm borealc, Solidago caesia, while Physocarpits opuli- 

 folius and Jiinipcrus virginiana are found occasionally on the cliff 

 face in rock pockets. 



Plants of the crevices and rock pockets : The character- 



istic plants of the rock crevices and smallest rock depressions 

 (figure 4), apparently growing out of the precipice itself, are : 



Dryoptcvis marginalis ^ Aspleniitm Trichouianes^ Polypodluni vidgare^ 



w 



Cystopttvis fragilis^ Woodsia ilvejisis^ IV. obtiisa^ Aqidlegia cana- 

 densis^ Cerastinm arvense var. oblongifolinm^ Arabis lyrata^ A. 

 laevigata^ Geraniion Robertiannm^ Heuchera aniericana^ Verbascum 

 Thapsus^ and Canipanfila rotnndifolia. The species seize every 

 vantage ground and it is surprising to find how their roots can pen- 

 etrate and hold fast to a narrow crack, or fissure, in the face ot 

 the smooth rock faces. 



WEED VEGETATION 



Native migrants: The native plants which have left their 

 .original habitats and have spread along the roadside form an ele- 

 TTtnent of the vegetation which illustrates what species have adapted 

 '.themselves to the new environmental conditions. The species of 

 •composite plants owe their spread to the light fruits which are 

 wind-distributed. 



*The writer wishes to state with reference to the ideutification of the plants men- 

 iioned in this and other of his phyto-geographic papers that the following plan is pur- 

 sued. If he feels no doubt as to the identity of the plant, it is noted in the field note- 

 book ; if he is in doubt, a specimen is collected, tagged, and identified at field head- 

 quarters by means of some manual • and, if unable after such study to name the plant, a 

 fpecimen is dried for more careful study with herbarium material at hand. 



