PAPERS ON BIOLOGY AND AGRICULTURE 169 



phytic direction. The latter of these two species is a mark of 

 attained mesophj-tism generally, while the former, though 

 indicating an increase in mesophytism usually, nevertheless 

 occurs in so \\ide a range of conditions as to be a 

 member of many stages, or degrees of stages, of the suc- 

 cession. All the phases, until a low beginning fonn of the 

 climax itself is reached, appear to be variations on the oak 

 representation. The occurrence of such species as Fraxinus 

 americana and Juglans, while representative of advanced 

 soil moisture, are probably less representative of a definite 

 stage of succession, though generally present in the climax. 



A deviation from the regularity of the succession is 

 shown by the association at the foot of a limestone ridge 

 capped by sandstone on the east shore of the Rock just 

 above Oregon, and consequently with a western exposure. 

 The representative tree species here are Quercus alba, 

 Q. ellipsoidalis, Hicoria ovata, Juniperus virginiana, and 

 infrequent specimens of Quercus macrocai^pa. Since the 

 ground cover includes such herbaceous forms as Monarda 

 stricta, Melilotus alba, Rudbeckia hirta, Desmodium illi- 

 noense. Cassia chamaecrista, the assemblage may be taken 

 to represent a response to xerophytic conditions on a 

 rather dry western exposure. It appears to be evident 

 from the surrounding growth that the xerophytism here- 

 abouts has been and is still decreasing in areal extent. It 

 is to be noted that for a number of years past the vege- 

 tation conditions have not been interfered with materially. 



A frequent response to the changed conditions afforded 

 by a stream that contains water at least the greater portion 

 of the time is shown by the appearance of Juglans nigra 

 and sometimes Gleditsia triancanthos in a stand of Quercus 

 ellipsoidalis, Q. velutina, Hicoria ovata, Quercus alba. An 

 herb such as Podophyllum peltatum is frequent. Migrants 

 like Acer saccharum, Hicoria minima, Celtis occidentalis, 

 indicate the direction in which the succession is going. A 

 good example of this is to be seen in a woodlot east of 

 the Rock opposite Oregon. Throughout the Rock River 

 region Hicoria minima is an excellent indicator of good 

 soil moisture. Celtis occidentalis, on the other hand, while 

 often occurring correspondingly, as a seeming indicator of 



