Minnesota Plant Life. 



437 



A variety of contrivances have been devised by plants to 

 retrench their evaporation. A simple one is the reduction of 

 the evaporating surface. Thus, leaves become small or are 

 altogether abandoned, as in the cacti and glassworts, or the 

 leaves may become thick and succulent, as in the purslanes and 

 claytonias. Sometimes the skins of the leaves are greatly thick- 

 ened and the air pores are reduced in number, as in the leather- 

 leafed wintergreens and heaths. Often the margins of the 

 leaves are rolled in so as to cover the air pores and protect them 

 from the rays of the sun, as in many prairie grasses, or in the 

 crowberries. Sometimes the leaves are covered with scales or 

 scurf, as in the buffalo-berries. Sometimes strong ethereal oils 

 are produced. These form a "scent-vapor-sheath" around the 

 plant, and thus temper the rays of the sun. For such a reason 

 many desert plants are strongly perfumed, as are wormwoods 

 or sage-brushes. The positions of the leaves upon the stem, 

 and their shapes, are often automatic, regulative devices, con- 

 nected with the evaporation of moisture. Good examples of 

 leaf-position unfavorable to rapid evaporation are furnished by 

 the cat-tails, flags and sweet-flags of marshy places. In these 

 the ribbon-shaped leaves stand erect and their surfaces are not 

 exposed to the strong illumination of the sun. Hence the 

 evaporation is slight. 



Electricity and magnetism. The adaptations of plants to 

 the forces of electricity and magnetism are not well understood, 

 nor have they yet been fully studied. It has been suggested 

 however, that points on leaves, and spines or thorns, may in some 

 instances be devices for the collection of atmospheric electricity. 



The soil or substratum. The relations of plants to the soil 

 are somewhat various. The texture of the soil may be either 

 loose or firm. Good examples of loose soil are furnished by 

 sand dunes, and there are a variety of special sand dune plants, 

 the underground parts of which, meeting with little resistance, 

 branch copiously in every direction. The low fertility of drift- 

 ing sand makes such a broad expansion of the root area essen- 

 tial, and as a consequence the plants that are able to grow 

 on sand dunes commonly bind the sand by their extensive root 

 systems, and may even, if they have become sulliciently estab- 

 lished, stop its drifting. In very resistant soils, such as hard 



