IOWA ACADEMY OF SCIENCES. 29 



to those already listed as characteristic of the rock exposures 

 {I. c p. 73) there were added the following species: 



Schedonnardus texanus Steud. Common. 



PotenWla pennsylvanica var strigosa Lehm. Not common. 



Gilia linearis Gray. Common. 



Euphorbia obtusata Pursh. Rather common. 



The lichens enumerated on pp. 74-5 (1. c.) should also be 

 transferred to this list, as all of the species were collected in 

 much greater numbers upon the most exposed portions of the 

 outcrops, and as comparatively few specimens are found upon 

 the straggling bowlders over the surrounding prairie, they are 

 rather characteristic of the rock exposures. Upon the more 

 pronounced soil which runs in and out among the exposures of 

 rock from the adjacent prairie, is a still different flora made up 

 of species which are peculiar to dry prairies, or which from 

 their .ready adaptability to circumstances are often found upon 

 them The species which have not already been listed {I. c. pp. 

 74-5) are enumerated in Table II. 



Surrounding the pond and pools, or growing in them, were 

 representatives of still another flora consisting of aquatic and 

 marsh species, most of which are common eastward. The 

 additional species of this group are reported in Table III. The 

 first and fourth groups represent an eastern flora, and the sec- 

 ond and third groups a flora more nearly western in its rela- 

 tionship {I. c. p. 76). 



We have here, then, four distinct floras meeting upon a very 

 restricted area. The lines between them are sharply drawn 

 (excepting perhaps that between the second and third groups), 

 and the collector need but take two or three steps to pass from 

 the aquatic and alluvial flora of the east to that of the dry 

 plains of the west. 



A general comparison of the plants and of the conditions 

 existing in June and August* brought out the fact that the pre- 

 vailing plants of the region which belong to the eastern flora 

 flowered and matured earlier in the season while the influence 

 of the spring rains was still abundantly felt, while those which 

 are more characteristic of dry grounds reached their develop- 

 ment {i e., flowered and fruited) later in the season after the 

 drier summer period had fairly set in. 



All this, of course, emphasizes the well-known fact that 

 most plants are more or less restricted in their choice of hab- 



*The plants discussed in 'he first paper were collected in August. 



