38 
IOWA ACADEMY OP SCIENCE 
The American plum (Primus americana) formed thickets and 
hence the percentage runs, high ; these thickets occur on the edge 
of the timber. On the bottoms the black walnut (Juglans nigra) 
is quite unevenly distributed, in some cases making solid blocks 
of this species alone. The soft maple (Acer saccharinum) was 
mixed with the green ash (Fraxinus pennsylvanica var. lance- 
olata) and occurred on the banks of sloughs. On the shores of 
the stream was an abundance of black willow (Salix nigra) and 
(Salix amygdaloides) . The more important undergrowth of the 
bottom consisted of blue grass (Poa pratensis), sneezeweed (He- 
lenium autumnale), smartweed (Polygonum lapathifolium var. 
incarnatum) , ironweed (Vernonia fasciculdta) (Acnida tuber- 
culata), northern nut grass (Cyperus esculentus) and such weeds 
as bull thistle (Cirsium lanceolatum) and shepherd’s purse 
( Gapsella Bur sa^past oris ) . 
The distribution of trees in the region is most interesting. 
The trees are most abundant in the sheltered valleys. The bluffs 
of this region from which trees are absent are such as are ex- 
posed to the west and southwest. The tree growth is often 
sharply limited where wind and sun have free access. This con- 
dition is strikingly shown in figures 1 and 2, plate 33, given 
by Shimek in his paper on the Geology of Harrison and Mo- 
nona counties. 
It does not, however, follow that all of such protected areas 
have timber. This has been discussed by Shimek in the paper 
referred to, in which he says, ‘Ht is now necessary to consider 
the application of these results to our problem. The prairie 
areas are uniformly so situated that they are fully exposed to 
the factors which cause rapid evaporation, namely the sun and 
the wind. During much of the year they may present conditions 
quite favorable to plant growth, but there are seasons and there 
are portions of the- year, especially in midsummer, when evap- 
oration and consequent desiccation become so extreme that only 
those plants which are especially adapted to dry regions can 
survive. The more or less frequent recurrence of such periods 
which are fatal to the mesophytes of the forest is sufficient to 
wipe out or rather prevent the development of a forest flora on 
those surfaces which are most exposed to evaporation. Forest 
trees are perennial and must exist through all the varying condi- 
tions of succeeding seasons. Any period, no matter how short, 
which is fatal to trees is sufficient to prevent the development 
