520 GLEASON AND MCFARLAND: INTRODUCED VEGETATION 
able to penetrate, in general, less than twenty meters into the aspen 
thickets. Even yet they are most numerous in the quadrats 
nearest the roadway, as shown by the following table, in which 
the data for the one hundred series have been collected. The 
first figure after each species indicates the number of first quadrats 
in which it occurs, the second figure the number of second quadrats, 
and so on. 
CREATES ie hr he ach oes |.65| 40| 29) 29|17| 11 4| 2405 
PO PRUIEN SIG Ce ais a io ae ia yl ee he | 63 28, a9) 20105 | 7-412 
ERIS GUE I GY hy Pik ia be Die ke 59| Io} 8} 8) 10] 3 r | 
IPRIPR Wm DIGIENSE 7h es ei ee | 40} 4} of 4| 6] '3| 2 | 
PPT OUUTE SEDENS he Oe cn ee ee pat} WSs 6) S| alee ae 
PPM OMUME DYOLENSE ec ea i Ooty. see ON SS | 16 Cees eee Po a ae cee | r| 
HERNAN CLOSED: ee OO gt ON | Tat: Ol t8\ eclas Veal tol “base 
A BrUPVrON repens: 6k r4| 6 2| 3| 2| 2 | 
TefouUum NVC ANI: os Ob Biers or | | 
Léepidium virginicum: > oo 0 oe Oe Rae aes Tie a a oa oe a 
OCIS ‘IOWMET ONE Se Se Ns es wh we, ate 3| 4 eee | 
Taraxacum officinale.......... Be La ee ae fees 3 | a | 
Chrysanthemum Leucanthemum var. pin-| | | | 
rite 7g SMe eae) Gm iat acy tet ge ae a ee | | | 
Verbascum Tha pee ie ct toe Sy es | | | ahs I | 
Total. Brie a meta NENG TUS rele oraroNae tn ae boasb 114 104, 87! 61/30 | 30 12 trol F,. 
These figures ae indicate the close dependence of the 
introduced species upon human operations. Besides these four- 
teen species, Avena sativa, Polygonum aviculare, Cerastium vul- 
gatum, and Achillea Millefolium also occur in the same association. 
deep ravine traverses a part of the aspen region, and some 
springs at its bottom make it a favorite camp ground and watering 
place for horses. In the small flat clearing at the bottom of this 
ravine fourteen introduced species occur, including besides those 
listed above Secale cereale, Arenaria serpyllifolia, and Cirsium 
lanceolatum. In all, 21 introduced species are known to occur in 
the aspen region. 
Only three species of the introduced group are apparently 
establishing themselves in the aspen association, in spite of the 
open vegetation and the abundant space available for plants. Of 
these the most successful is Rumex Acetosella, now found widely 
scattered throughout the association, with a frequency index of 
from 10 to 15. Lepidium virginicum is common along most 
roadsides, and in some places these colonies occur away from the 
