DISCUSSION 



93 



five of these 74 species are restricted to the 

 McLeansboro coal beds. The two new 

 genera which were also present in some of 

 the Carbondale coal beds, and are not 

 described (see pages 64 and 73), are rarely 

 found in the McLeansboro beds. 



DISCUSSION 



This investigation was primarily an ex- 

 ploration of the small spore content of the 

 various coal beds in Illinois and of the value 

 of these spores in the correlation of coal 

 beds. But in addition, much information 

 was obtained concerning the plant succes- 

 sion from bottom to top of a number of coal 

 beds, and the influence of geographic distri- 

 bution of samples of a particular bed on the 

 abundance variation in spore population. 



The causes of geographic variations in 

 abundance of the spore population are not 

 understood. Ecological factors such as 

 climate, physiographic features, or regional 

 factors may have been important. Among 

 the climatic factors which might produce 

 lateral variations in plant population are 

 differences in temperature, atmospheric 

 moisture, and light, but these are thought 

 to have been relatively uniform. The direc- 

 tion of prevailing winds is unknown, and 

 changes in direction and strength during 

 the time of coal accumulation might have 

 been important. Among physiographic fea- 

 tures which appear more likely to influence 

 the floral composition are topographic 

 variations, especially toward marginal areas. 

 A regional factor of importance might be 

 changes of humic accumulation in the soil. 



Bench zonation of particular genera and 

 species in certain coal beds points to plant 

 succession. 8 The ecesis 9 and migration of 

 plants upon Pennsylvanian soil (underclay) 

 may have been rapid. It is not yet known 

 whether or not the primary plant invaders 

 were herbaceous or arborescent forms. The 

 restriction of some genera and species to 

 either the upper or basal portions of coal 

 beds points to possible primary and climax 

 communities. 



8 The replacement of a group of plants, which occupy 

 a common habitat, by another group of plants (Nichols, 

 1923). 



9 The actual establishment of a plant in a new location. 



Summary 



Nineteen genera and 130 species of spores 

 have been identified from the coal beds in- 

 vestigated from 47 counties in Illinois. 

 Five of the genera and 100 of the species are 

 described in this report. These new genera 

 and species together with forms previously 

 described provide a working basis for the 

 correlation of the coal beds of Illinois. 

 Twenty-eight species are at the present 

 known to be restricted to one or another 

 single coal bed, 16 species appear to be 

 restricted to two coal beds, and 23 species 

 on the basis of present information are re- 

 stricted to three coal beds. In addition to 

 these restricted species several genera and 

 many species have significant ranges which 

 are invaluable in correlation studies. 



The vertical distribution of isopores, 

 microspores, and prepollen in Illinois coal 

 beds as observed in this investigation pro- 

 vides a means of specific identification of 

 coal beds. Correlations have been made and 

 are indicated in the text. 



ACKNOWLEDGMENTS 



The author wishes to express his gratitude 

 to G. H. Cady, Senior Geologist and Head 

 of the Coal Division, for his continued 

 interest, and to M. M. Leighton, Chief of 

 the Survey, for his support, which has made 

 this investigation possible. J. M. Schopf, 

 United States Geological Survey, has con- 

 tributed helpful suggestions concerning the 

 morphology of the winged pollens. Grate- 

 ful acknowledgment is due G. H. Cady and 

 L. R. Wilson, of Massachusetts State 

 College, for their comments and the reading 

 of the manuscript. Theodor Just of 

 Chicago Natural History Museum has 

 kindly checked the construction of new 

 scientific names proposed in this paper. The 

 aid of assistants and various members of 

 the Coal Division of the Survey in the col- 

 lection and preparation of coal samples is 

 gratefully acknowledged as well as the 

 typing of the manuscript by Mrs. R. C. 

 Honea. 



