306 THE STUDY OF PLANT COMMUNITIES " Chapter XI 



studied. Samples must be taken with care to prevent contamina- 

 tion, and several types of augers have been devised for the pur- 

 pose. With these, cores can be cut that, placed end to end, form a 

 continuous column of material for the entire depth of the deposit. 

 Borings are made in summer under most conditions, but, since it 

 is desirable to have them from the deepest part of the depression, 

 it is often advantageous to make them in winter from the frozen 

 surface. 



Identification and counting of the pollen grains must be done 

 under a microscope. This necessitates treatment of the samples 

 with one of the several methods recommended 48 to eliminate for- 

 eign material and to concentrate the grains. Identification is facili- 

 tated by reference to illustrations 274 and by comparison with a 

 series of grains taken from modern plants. What constitutes an 



NORTHERN MtNNESOTA 

 AND WISCONSIN 



NORTH-CENTRAL 

 STATES 



NORTHEASTERN 

 OCEANIC 



FlG. 164. Schematic pollen profiles that show the general picture of what 

 is known of vegetational history for the eastern United States. F— fir, G— 

 grassland complex, H— hardwoods except oak, O— oak, P— pine, S— spruce. 

 Depth shown vertically, percentages horizontally. Although there are differ- 

 ences relatable to continental and maritime climates, there is regional similar- 

 ity in the indications of a middle warm, drier period, and the suggestion of 

 subsequent cooler, moister conditions leading into the present, as well as the 

 shift toward early proportions of species in the upper portions of the dia- 

 grams. Succession may be a factor in these latter shifts.— After Sears. 2 



219 



