434 Minnesota Plant Life. 



tical position. This is true of leaves in the compass plant and 

 many leaves on a variety of herbs. In some of the Austra- 

 lian blue-gum trees the leaves stand with their edges ver- 

 tical, and similar positions are very often maintained by 

 grasses of the prairie. When a plant has adopted the mat 

 habit of growth, indicating the absence of shade around it, it 

 commonly shows very small leaves, and no Minnesota mat 

 plant, lying exposed to the full glare of the sun as it does, has 

 large leaves. 



Moisture. The adaptations of plants to moisture are vari- 

 ous. Some plants live quite submerged in water, and others find 



FIG. 218. " Gallery woods," near Minnesota Falls, valley of the Minnesota, in the prairie 



district. Dependence of trees upon moisture is illustrated by their grouping 



in declivities. After photograph by Professor R. D. Irving. 



their most congenial home in deserts, or on the surfaces of inhos- 

 pitable rocks ; while between these two extremes of station there 

 are a great number of intermediate conditions worthy of care- 

 ful and extended investigation. All plants need some moisture. 

 Usually this moisture is absorbed by a special area of the plant 

 known as the root system. But leaves are, in some varieties, 

 able to absorb moisture, and aquatic plants characteristically 

 absorb over their whole surface. There are sometimes pres- 

 entations of liquid to the plant which it finds undesirable. For 

 example, foliage guards itself by a variety of devices against 

 the heavy rains of the tropics. Many of the same contrivances 



