TREES OP THE MISSOURI BASIN 
41 
Valley. The Quercus acuminata is. hardy as far north as Boone 
county in central Iowa and Allamakee county in northeastern 
Iowa. Both of these places have climatic conditions much more 
severe than has Fremont county. 
It is evident that there are other factors which enter into 
the distribution of trees. What all of these factors are has not 
been determined. 
THE ORIGIN OF THE TREE FLORA. 
A discussion of the trees found on the Missouri slope is of 
interest, especially when we consider their abundance and origin. 
The family Aceraceae contains three species, the soft maple 
{Acer saccharinum) , the box elder {Acer Negundo) and black 
maple {Acer nigrum). These are common throughout the re- 
gion. The hard maple is a northern invasion reaching the Spirit 
Lake and Okoboji district and as far south as Buena Vista 
county along the Little Sioux. Of the family TJlmaceae^ the 
American elm {TJlmus americana) and slippery elm {TJ. fulva) 
are generally distributed. The corky bark elm {TJlmus Thom- 
asii) is rare, reported only from the Little Sioux in Buena Vista 
county by Pammel and again in Harrison county by Shimek. 
It should occur in intervening counties. 
The hackberry {Celtis Occident alis) is quite generally distrib- 
uted. Shimek refers the form in upland woods to var. crassi- 
folia, a western form. There are three ashes of the family 
Oleaceae in the region. The common white ash {Fraxinus amer- 
icana) is a southern species which is fairly common in Fremont 
county but becomes less frequent near Council Bluffs, though 
reported by Macbride from Osceola, Dickinson, Sac and Ida 
counties (probably an error). From remarks made by Macbride 
under the species it is probable that the species recorded by him 
is F. pennsylvanica and the variety lanceolata, as they are com- 
mon in the Spirit Lake and Okoboji region. ^ 
Of the rose family {Rosaceae) the choke cherry is general in 
the region from Fremont to Lyon counties. The same is true 
of the common plum (P. americana) or at least what is usually 
referred to this species by botanists. The rum or black cherry 
(P. serotina) is southern, more common in Fremont and Mills 
counties than in Pottawattamie. Some of the trees observed 
were of fairly good size. It is possible, of course, that it is an 
introduced species. The P. Ghicasa was observed nowhere wild. 
