392 H. Engelmann on vegetation known as 
tion. The same cause prevents the parched surface from draw- 
ing upon the substrata for a supply of humidity from the deep 
seated permanent water-level; and moreover it is generally 
assumed that, whenever the depth of the water-level exceeds a 
few feet, it can exercise a comparatively small influence upon 
the humidity of the soil in the dry season, during which the 
main supply of water for the vegetation seems to be absorbed by 
the leaves and by the soil directly from the atmosphere. e 
excessive humidity of spring is therefore followed on the flats 
by an excessive drought, almost without a congenial growing 
season between. The closely packed condition of the soil 
presents also an obstacle to the deep-penetrating roots, and 
the latter are not invited to any exertion in that direction, 
ecause the same cause prevents the access of oxygen an 
of other elements of nutrition. A heavy rain which would 
ing deep; it will soon be 
dried by sun and wind, and will suddenly chill the soil, 
without exerting to a large degree the beneficial influences which 
it would have on a more favored soil. 
While the former vegetation of the prairies seems to have 
been one suited to wet ground, the flats in their unimproved 
state will only sustain one which is able to outlive the excessive 
wet of the spring season, and the sudden change to the dryness 
of summer and fall. The trees which grow there are such as 
have shallow spreading roots, and can withstand considerable 
drought; deeper rooted ones would be killed in the spring, 
more delicate plants would wither under the scorching sun of 
“summer and fall if they did survive the spring. Other causes 
which determine the vegetation of the flats may be found prin- 
cipally in the chemical properties of the soil. 
ch is the condition of the typical flats in their uncultivated 
state, where they present their worst features; there are how- 
ever gradations to the better, and they are by no means as un- 
productive as it might appear. They were so, indeed, under 
the old system of pioneer cultivation, which expected nature to 
do everything, and man as little as possible; but if the soil is 
opened by deep cultivation, and vegetable mould is created, and 
the soil loosened by ploughing under green crops and manuring, 
it will be found to improve steadily as soon as the roots have 
once began to penetrate it. The first crop is frequently quite 
