NATIVE WOODY PLANTS OF THE UNITED STATES 27 
Average annual frost-free season 230 days in the extreme north- 
east to more than 260 days in the south-central and southwestern 
portions. Snow cover rare. 
REGION 31, SOUTH CENTRAL FLORIDA 
Subhumid, mesothermal, with rainfall adequate at all seasons. 
Killing frost likely on the average in half the years; frost-free season 
well over 260 days. Snow cover extremely rare. 
REGION 32, SUBTROPICAL FLORIDA 
Humid, tropical, with adequate rainfall at all seasons. In the 
north, killing frosts are apt to occur in half the years; in the south 
there is no record of a killing frost. 
NITROGEN-FIXING BACTERIA 
Technicians engaged in planting for erosion control often very 
wisely choose species that are able, by means of a symbiotic relation- 
ship with a species of Rhizobium to enrich the soil by the addition 
of certain nitrogen compounds. With some exceptions, such plants 
belong to the Leguminosae. Because the soils left after considerable 
washing has taken place are often poor and unproductive, the in- 
corporation of nitrogen is of great importance in their reclamation. 
Lespedezas, alfalfa, vetch, clover, acacias, black locust, and Scotch 
broom are legumes and have been advocated for use particularly on 
account of their root associations with nitrogen-fixing bacteria. 
The decay of various parts of the legume as well as of the root 
nodules housing the bacteria results in the addition of nitrogen com- 
pounds to the soul. Vetch, peas, beans, clover, and other annual plants, 
or those on which nodules are produced each spring to develop 
through the season and decay in the autumn enrich the soil to the 
extent of about 100 pounds of nitrogen per acre annually if the crop 
is plowed under. In black locust, Sophora, some species of Acacia, 
and, others, the nodules may last for more than one growing period, 
and often become woody. Part of the time during the life of the 
nodules the nitrogenous compounds may be used by the plant. 
Not only do members of the Leguminosae form these symbiotic 
relationships, but they are formed also by shrubs and trees of the 
genera Alnus, Ceanothus, Hlaeagnus, Shepherdia, and Myrica, besides 
members of the Cycadaceae and Podocarpaceae. Members of the lat- 
ter two families will, of course, rarely be used to any great extent 
in planting in the United States. 
The nodules formed in nonleguminous plant roots are by some be- 
heved to be caused by species of Rhizobium, although this is denied 
by others. But that the nodules of nonlegumes play a part in the 
fixation of atmospheric nitrogen very similar to that performed by 
legume nodules appears to be fairly well established. Cross-inocula- 
tion of legumes with strains from Alnus and EHlaeagnus is claimed 
to have been accomplished successfully, but ordinarily certain strains 
or races are specific for the host-plant species. Inoculation of plant- 
ing sites with the proper strain is an established farm practice 
although the bacteria are probably distributed as wind-borne dust 
