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. 



EEGION 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, STIBTBOPICAL 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, oy means of a symbiotic relation- 

 ship with a species of RMftobvum 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 soil. 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, Elaeagnus, Shepherdia, and Myrica, besides 

 members of the Cycadaceae and Podooarpaceae. 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- 

 lieved 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 w y ell established. Cross-inocula- 

 tion of legumes with strains from Alnus and Elaeagnus 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 



