162 IOWA ACADEMY OF SCIENCES. 



Gymmocladus Canadensis . Juglans nigra. 



Morus rubra. Rharnnus lanceolata . 



Neluntbo lutea. Vernonia Noveboracensis . 



Polygonum Pennsylvanicuni. Polygonum dumetorum var. scandens. 



Martynia proboscidea. Eupatorium, serotinum . 



Arid transition. This area reaches its greatest develop- 

 ment along the immediate border of the Missouri river on 

 the loess bluffs but extends eastward to the divide between 

 the Mississippi and Missouri rivers in Carroll and Dickinson 

 counties. Representative plants are as follows: 



Cnicus canescens . Shepherdia argentca (N). 



Symphoricarpos occidentalis . Helianthus annuus. 



Yucca augustif alia . Helianthus Maximiliani. 



Petalostemon rnultiflorus. Gaura coccinea. 



Aplopappus spinulosus . Gaura parviflora . 



Grindelia squarrosa . Liatris punctata. 



Euphorbia marginata. Euphorbia heterophylla. 



Hosackia Purshiana. Lactiica pulchella. 



Erysimum asperum,. Dalea laxiflora. 



Psoralea esculenta. l\Ientzelia ornata. 



Lygodesmia juncea. St)orobolus cuspidaius . 



Bouteloua oligostachya. Buchloe dactyloides (N. W.). 

 Schedonnardus Texanus [^ W .) ■ Oxytropis Lambertii. 

 Astragalus lotiflorus var. brachypus . 



It should be observed that the zonal boundaries of plants 

 are not sharply marked, but that the different areas con- 

 tain marked types of each of the areas. The main features 

 of the flora is essentially prairie. The intermingling of 

 western and eastern prairie types is most marked on the 

 loess bluff's. 



RAINFALL. 



That moisture is an important factor in the development 

 of plants cannot be questioned. The occurrence of strictly 

 western plants within the border of Iowa must in part be 

 attributed to the smaller amount of precipitation. The 

 precipitation is given for the same points as the temper- 

 ature. 



