584 THE FLORA OF THE PRAIRIES. 
bottoms in unsurpassed perfection. "Though they are all, or 
nearly all, of the same species (Prunus Americana Marsh), 
the varieties in respect to the form, size, color and quality 
of the fruit are almost endless, the plums varying in form 
from spherical to egg-shaped, and from nearly white through 
every intermediate stage of color to yellow and even dark 
red, and in flavor from bitter, uneatable kinds to those as 
delicious as the highly cultivated varieties of the garden. 
From the abundanee of woody climbers the forests of the 
river bottoms sometimes present an almost tropical aspect. 
The Virginia creeper (Ampelopsis quinquefolia Michx.), and 
the winter grape ( Vitis cordifolia Michx.), climb to the tops 
of the highest trees, with a diameter of the stem exceeding 
any specimens I have elsewhere seen. Other climbers are 
frequent, including the singular wild cucumber, or balsam 
apple (Echinocystis lobata T. & G.), which assumes an al- 
most tropical luxuriance, here and there abundantly envel- 
oping the trees. 
The restriction of the forests to the river bottoms and 
their banks has previously been alluded to as a remarkable 
feature, of which various explanations have been offered. 
The fact of the rapid encroachment of the forests upon the 
prairies wherever they have been protected from exposure 
to the annual fires that formerly swept over the country, and 
the rapid growth of the timber whenever it becomes estab- 
lished, indicate clearly that not only have the fires had much 
to do with their restriction, but that there is nothing either 
in the climate or the soil unfavorable to their rapid spread. 
The damper northern slopes of the streams being also gen- 
erally better wooded than the necessarily drier southern 
slopes, also points to the fires as the great agency that has 
operated through long ages to check their increase, and that 
their circumscription has had little to do with the peculiar 
origin of the prairies and of their present flora, as some 
have formerly supposed. 
As has been already incidentally remarked, the vegetation 
