738 METASPERMAE OF THE MINNESOTA VALLEY. 
IV. EXAMINATION OF PHYSIOGNOMIC CHARACTERS OF THE 
METASPERMIC PLANTS OF THE MINNESOTA VALLEY. 
The plant-physiognomy of any district is determined by the 
habits and habitats of its plants. Without entering upon ex- 
haustive analyses of the principal physiognomic groups—the 
forest and the prairie—it will be possible, nevertheless, to iso- 
late three groups of species of characteristic habit and three 
others of characteristic habitat. The first three elements will 
then be as follows: 
A. The Arboreal element. 
B. The Shrubby element. 
C. The Herbaceous element. 
Evidently transitional forms must be recognised between 
these groups, but in general aclassification may be attempted of 
all species into one or another of the groups themselves. The 
character of a principal woody trunk is considered to indicate 
the tree, if this coexists with a sufficient size. The woody 
character without the principal trunk is deemed characteristic 
of the shrub, and the absence of a distinctly woody stem is con- 
sidered characteristic of the herb. Of course all Metaspermee 
contain woody tissue in greater or less abundance. If, how- 
ever, the cambium cylinders are not developed, the plant is 
generally characterised as herbaceous. 
Of the three habitat elements the classification may be as 
follows: 
A. The Aquatic element. 
B. The Swamp and Marsh element. 
C. The Drier-soil element. 
As before, there are transitional forms between these groups 
and the entry of a given species may be difficult. Indeed in 
the same species certain individuals may be aquatic, and others 
may be found in more terrestrial localities. As in the case of 
the habit elements there is, then, some difficulty in obtaining 
arigid classification. In the following tables two elements are 
unlisted—the herbaceous and the drier-soil elements. This is 
because these elements are in the nature of residua and may be 
understood closely enough from the other four elements that 
are listed. The following table gives a list of arboreal plants 
found growing spontaneously and indigenously in the valley of 
the Minnesota. 
