HARSHBERGER: PHYYO-GEOGRAPHIC SKETCH 155 
Fields (6) that are abandoned after a long period of cultiva- 
tion and in which weeds have been permitted to grow rampant 
and in which trees begin to appear, show some interesting changes 
in the flora which are not mentioned in connection with fields of 
the first category. In such fields, trees appear not from the stump, 
but as chance seedlings, that grow from seeds carried by the winds 
or animals. The predominant weeds noticed on an old dam site 
along Crum creek were Eupatorium purpureum .., Aster (several 
species), Solidago (several species), Rubus (several species), Chelone 
glabra L., Ambrosia trifida L., Rudbeckia laciniata L., etc. These 
were surrounded by small trees of the following species : Betula 
lenta L., Liriodendron Tulipifera L., Acer rubrum L., Ulmus 
Americana L. and Fraxinus Americana LL. All of these trees 
have winged fruits and without doubt the trees grew from seeds 
Wafted to the open dam site by the wind. 
_ RoapstpE-pLaAnT FORMATION.— The flora of a roadside is 
peculiar, Along the highways we find a miscellaneous assort- 
ment of plants, mostly weeds, with an admixture of native plants 
that are able to compete with foreign introductions and that have 
adapted themselves to growing under the more trying conditions 
of the open, shadeless, dust-laden environment. The weeds of 
the roadsides in southeastern Pennsylvania have been derived from 
two main sources, viz., Europe, and eastern North America. The 
European weeds seem to get the upper hand in the struggle for 
existence for several reasons: (1) Because they have been longer 
associated with man and his methods of cultivation ; (2) because 
they have adapted themselves during a thousand years, since 
Europe was a forest wilderness, to living in the open, in fields 
along roadsides, and in garden patches ; (3) because in migrating 
to a new country they have been removed from their competitors 
by whom they were held in subjection through the. struggle for 
existence ; (4) because in eastern North America they come into 
Competition with native species mostly derived from the forests and 
that have not had sufficient time to adapt themselves to the 
changed conditions produced by the destruction of the forests, 
the cultivation of the soil and growth in open places subjected to 
the full force of the sunlight. 
estern American weeds, seem to have an advantage, because 
