1905] BROWN—THE PLANT SOCIETIES AT YPSILANTI 277 
OTHER BIOLOGICAL GROUPS. 
The account thus far deals mainly with indigenous plants which 
seem to bear a more or less constant relation to the environment, 
and includes a total of 316 species which have been classified into 
societies. There remain eighty-four species found upon the area 
surveyed, which occur in widely varying situations. These have 
been classified into two groups. 
1. THE HETEROPHYTES.—A group of twenty-one native species 
were found to occur in so wide a range of habitat as to be best termed 
heterophytes. The most common of these are Onagra biennis, 
Taraxacum Taraxacum, Poa pratensis, Aster novae-angliae, and 
Solidago canadensis. 
2. INTRODUCED WEEDS.—A second group, the exotics, embracing 
the introduced weeds, is characterized by a similar wide range of 
habitat. Their presence is due solely to the agency of man, and 
serves to indicate the extent to which he has changed the composition 
of the original vegetation by clearing away the forest and cultivating 
areas in the vicinity. Nearly one-seventh of all the species collected 
at the bayou have been introduced. Of these fifty-two are European, 
four (Amaranthus retroflexus, Chenopodium ambrosioides, Mollugo 
verticillata, Datura Tatula) are tropical American; one (Ailanthus 
glandulosa) Chinese; one (Lonicera Tatarica) northern Asiatic; one 
(Abutilon Abutilon) southern Asiatic. The important fact is brought 
neat in the study of the bayou vegetation that the aquatic and swamp 
Societies are almost free from introduced species. Only five species were 
found in this area: Solanum Dulcamara, Mentha piperita, Roripa 
Nasturtium, Salix purpurea, and S. alba vitellina. These were con- 
fined mainly to the drained swamp. The juniper-heath is also nearly 
free from exotic species. The largest number of exotic species occurs 
. ie grain-bearing or grass-producing societies, the highest pact 
age being associated with the black oak and the oak-hickory societies. 
PROMINENT INTER-RELATIONS. 
From the fact that members of a plant society live together under 
common conditions, it is to be expected that individual species of a 
8toup would possess common adaptations and hence resemble one 
