GRASSES OF IOWA. , 391 



4. The biotic determine the tension line between forest and prairie. 

 Plant main determines the extermination constantly going on. The 

 forest in our prairie state is constantly extending its area, as conditions 

 suitable for the species are made. In western Wisconsin the hills were 

 usually devoid of tree growth, but since the absence of fire, forest growth 

 has spread to the grassy slopes; the hazel and birch displacing the Andro- 

 pogons and Boutelouas; the hazel later being displaced by the white and 

 scarlet oaks. 



ECOLOGICAL CLASSIFICATION. 



Several different classifications have been proposed tor plant soeie- 

 rits. The classification of Warming is generally used in this connection. 

 The author has made the following groups: xerophytic, halophytic, 

 mesophytic and hydrophytic. 



XEROPHYTIC. 



The xerophytic grasses have leaves so constructed that the amount 

 of water transpired is reduced to a minimum. The leaves never open, or 

 roll up during dry weather. As an example, Festuca tcnella, may be given, 

 which grows throughout the state in dry soil. It is specially common in 

 western Nebraska, Colorado, Wyoming and South Dakota regions where 

 xerophytic plants are common. The Sporobolus cryptandrus and other 

 western species occur on sandy-gravelly soil, and high prairies subject to 

 drouth. Associated with this species we may mention Stipa spartea, ami 

 in the western part of this state along the Missouri, Sporobolus cusplda- 

 tus, Calamovilfa longifolia, Sporobolus longifolius, Bouteloua oligos- 

 t achy a locally, and Bouteloua raccmosa, more widely distributed. The 

 xerophytic grasses of eastern or central, and southern Iowa usually occur 

 on sandy and gravelly soils, in which there may be an abundance of 

 moisture at times, but as these soils are well drained, and easily give up 

 their water, they become very dry. Here we find such grasses as Bou- 

 teloua hirsuta. Aristida basiramea, Panicum capillare, Cenchrus tribu- 

 loides and Festuca tenella; in southeastern Iowa Trioda cuprea, Panicum 

 autumnale , Aristida oligantha and A. gracilis. Another most striking 

 peculiarity of xerophytic grasses is that they grow in bunches. A single 

 bunch many contain a hundred or more culms. These bunches are many 

 years old. Some one has suggested that the low places in these bunches 

 are formed for the purpose of providing the plant with water, as the 

 central part is hollow and the surrounding part elevated. There can be 

 no doubt that water collects in this way, but it is only incidentally that 

 plants have utilized this water. This formation is brought about by the 



