NATIVE WOODY PLANTS OF THE UNITED STATES 17 
will be found that the natural distribution, or the area of usual culti- 
vation, often does not coincide with the area that is shown on the 
map to be adapted to them. As a matter of fact, the growing of the 
plants may often occur beyond the area designated for them, or they 
may not be known over all the area suggested for them even though 
conditions are apparently satisfactory for their success. This over- 
lapping of adjacent areas may be only slight, but again it may extend 
much beyond the zone lines. 
It must also be recognized, however, that some plants will grow only 
under special conditions and that only in areas providing these con- 
ditions can they be expected to thrive. Bog, marsh, and water plants 
will grow only near or in water in whatever region they are found. 
Likewise plants adapted to dry knolls, ridges, or to sandy areas grow 
only in such locations, and those needing acid soils, alkaline soils, 
shade, or full sunlight can be expected to thrive only when those con- 
ditions are provided. Recommendations of plants having special re- 
quirements can be made for the different regions in the same way that 
plants thriving under more general conditions are listed, with nota- 
tions concerning their special requirements. 
When the 13-zone map was published it was recognized that some of 
the regions were too inclusive but for various reasons it was not. 
deemed desirable to subdivide them at that time. Studies were imme- 
diately begun, however, looking toward making the regions smaller 
and more informing without going into too much detail. The number 
of regions was doubled, but as the study progressed it became obvious 
that important differences in plant reaction and plant associations 
would be ignored unless more regions were included, so the number 
was increased to 29 zones, then to 30, and eventually to 32. 
The subdivision was begun by combining the known natural distri- 
bution of plants with regions of successful cultivation of trees, shrubs, 
and herbaceous perennials. Careful consideration was given to 
Weather Bureau maps of rainfall totals and distribution through the 
year and of temperatures, not only maximum and minimum but frost 
dates and length of growing season. These results were carefully 
checked with O. E. Baker, of the Bureau of Agricultural Economics, 
in relation to his crop-geography studies, and the resulting modifica- 
tions were later made in the growth-region limits. 
Subsequently the map was checked with the late Curtis F. Marbut, 
of the Bureau of Chemistry and Soils, and the zone lines were made 
to conform to the outline of soil types where there were no apparent 
discrepancies between the field studies and these modified outlines. 
Since the original publication of the 32-zone map in 1922 there 
have been many opportunities to consult with students having inti- 
mate knowledge of relatively local areas and all such opportunities 
have been utilized to check the accuracy of details. This has resulted 
in a few slight modifications. 
PLANT-GROWTH REGIONS 
FACTORS IN PLANT DISTRIBUTION 
There are three factors of great importance in the distribution of 
any given plant. Each may act separately, although they commonly 
operate in combination, They are climate, soil, and time. 
