FLOWERING PLANTS AND FERNS OF INDIANA. 561 



regions near Lake Michigan, it has had a very marked effect in deter- 

 mining the existing flora. 



It is the influence of these factors, operating in differing localities in 

 varying intensity and proportion, that determines plant distribution. 

 As conditions change, the plants must either change to meet these new 

 conditions or perish. Various adaptations, in familiar forms, show 

 how plastic is the plant and how quickly it responds to new conditions. 

 The hackberry (Celtis occidentalis), which is a tall tree on bottom 

 lands, becomes a thorny shrub on dry sand dunes, and the ordinary- 

 wild rose, which in swamps or upon a lake margin, is from six to ten 

 feet high and almost thornless, dwindles to a stunted bush from twelve 

 to eighteen inches high, and is absolutely covered with thorns when it 

 grows upon a dry sand ridge. Marked changes also show themselves 

 in the size and structure of the leaf. 



It is probable that water, in the long run, more than any other of the 

 factors just named, influences plant distribution, although it must be 

 remembered that no one factor alone determines a plant society. So 

 important is this water factor that the grouping of plants is generally 

 based upon it, the subordinate groupings showing the relation of the 

 plants to the other factors. Upon this basis three great groups of 

 plants can be recognized in Indiana. 



I. Water-loving plants (Hydrophytes) adapted for living either 

 wholly submerged in the water or in soil exceedingly rich in water. 

 Evidently such conditions are to be found in' the shallower water of 

 lakes, in swamp regions and along the lower stretches of our streams. 

 The plants growing in this extreme condition are characteristic and 

 easily recognized. The "cat-tail," the various "rushes," tall, rank 

 "sedges," "wild flags" and many other familiar forms will be at once 

 recalled as of this type. 



II. Dry soil plants (Xerophytes), which stand at the opposite ex- 

 treme of the water condition, successfully maintaining life in localities 

 where both soil and air are extremely dry. It is apparent upon the 

 most cursory thought, that plants under these conditions must differ 

 greatly in general appearance from the water-loving plants. To meet 

 their drought conditions, many adaptations have been developed, both 

 for the prevention of the loss of moisture from the plant and for stor- 

 ing away for future use the scant excess of water that may from time 

 to time be secured. Regions furnishing these conditions are not abun- 

 dant in Indiana, although in the "dune'' regions along the southern 

 shore of Lake Michigan and on occasional sand ridges, left by the 

 glaciers, such desert plants may be found. Bug seed (Gahile edentula), 



