738 METASPEKMAE 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 a classification 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 Metaspermae 

 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 diflicult. 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 

 a rigid 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. 



