TORRET, 
F January-February, 1931 - sheds 
Persistence of Exotic Plants Under Forest Conditions 
Rotanp M. HARPER 
Most weeds, whether introduced or supposedly native, are 
confined to places where natural conditions have been greatly 
modified by civilization, and that indeed is one of the best ways 
of identifying plants as weeds when their recent history is un- 
known.' When human interference ceases the usual tendency is 
for native plants to recover the ground from which they have 
been dispossessed. For example, in a cultivated field in mid- 
summer the only plants visible may be of a single cultivated 
species, that could not maintain itself in competition with weeds 
if those were not frequently hoed out or plowed under. In the 
fall, after the crop is harvested, the ground may become pretty 
well covered with introduced annual weeds, whose seeds have 
lain dormant during the summer. If the field should be left un- 
cultivated for several years, the first year after cultivation will 
see a different set of weeds, with a larger proportion of supposed 
native species, a few of them perhaps biennials or perennials. If 
ma forested region, trees gradually come in too, and by the 
time the trees have grown to average size, casting as much shade 
as is normal for that type of forest, all introduced weeds may 
have disappeared, and the forest may not be easily distinguished 
from a virgin one, except that its flora is likely to be somewhat 
poorer, lacking some of the rarer species.? 
; Within a mile or two of the locality to be described presently 
(in Lee County, Georgia), on the day the observations were 
made (November 9, 1930), the following annual weeds were no- 
ticed in fields from which a crop of corn or cotton had recently 
1 See Bull. Torrey Botanical Club 35: 347-360, 1908; 37: 117-120, 1910. 
_ ° It is said that wire-grass (Aristida stricta), one of the commonest herb 
in long-leaf pine forests in North Carolina, Georgia and Florida (scarcer in 
South Carolina, Alabama and Mississippi), never comes back on land that has 
once been cultivated. Though perhaps it would if given time enough. 
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