402 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
per cent appears at three stations, 28 per cent appear at two 
stations, and 63 per cent at a single station. 
The very distinct division of the flora into prairie and forest, 
with the great differences between their floras, naturally lead 
to attempts to explain their clcse proximity. Lists of plants have 
been made by various botanists within the territory surround- 
ing Lyon county on the north, the east and the south. In Mc- 
Millan 7 s “Metaspermae of the Minnesota Valley”, we have listed, 
among other things, the trees of that nearby tract. None of the 
trees in the Lyon county area are absent from these lists, nor 
from the lists in Shimek’s report on territory east of this, as 
given in “The Plant Geography of 'the Okoboji Region”. This 
Lake Okoboji region is in the valley of the Little Sioux river, 
and between that valley and the valley of the Big Sioux river 
intervene long stretches of open prairie without trees. If the 
forest trees of the Big Sioux valley had come from tho east 
across the country it would be reasonable to expect trees to be 
found on the intervening prairie. The same may be said con- 
cerning the introduction of forest trees from the Minnesota for- 
est to the northeast, A glance, however, at the map of Iowa 
makes clear a highway the trees could follow, viz. : up the valley 
of the Missouri river, and along the Big Sioux river to the 
region. All along this course the protection of bluffs and broken 
ground, the presence of water nearby, and the transporting 
agencies of animal life that would follow the stream, would be 
present to aid in the gradual advance of the trees into a region 
dominantly xerophytic prairie. Local conditions make it pos- 
sible for the trees of protected places to survive the generally 
harsh conditions that prevent the presence of a general forest. 
The prairie plants, in turn, are almost all found outside of 
Lyon county, and most of their names are reported from Har- 
rison and Monona counties. All of them occur in Woodbury 
county, about Sioux City. They extend north into Minnesota, 
as shown by the reports of Upham, and sixteen of them reach 
the Red river valley, as shown by the same author. Lyon county 
is near the eastern edge of the great plains and the plants are 
those of the dry prairies. They could be brought to this area in 
many cases by wind transportation of seeds, by seeds being car- 
ried by birds and animals, and by gradual advance by growth. 
Not needing the protection of a rh r er bank, the greater amount 
