28 MISC. PUBLICATION 303, U. S. DEFT. OF AGRICULTURE 



under natural conditions. The rotation of legumes with nonlegumes 

 for soil enrichment is also a common practice. In Taiwan (For- 

 mosa), according to E. Kent Beattie, alders are included in the rota- 

 tion of rice fields in much the same manner, presumably for the bene- 

 fit derived from the nitrogen-fixing bacteria of the shrub's roots. 



In general, plants producing such nodules are able to live in very 

 poor soils, since lack of nitrogen is not as great a factor in their 

 success as it may be for other plants. For this reason the leguminous 

 plants are particularly satisfactory for planting on poor, eroded 

 land, the soil of which will with difficulty support other plants. 

 Many legumes are unsuccessful on acid soils, doing better on neutral 

 or alkaline soils, where calcium appears to be of importance in the 

 nutrition of the bacteria of the nodules. This is not always true, 

 however, as in Lupinus. 



There are a number of exceptions to the rule that nodule forma- 

 tion caused by fihisobium occurs in all leguminous plants, Those 

 of interest here are Cercis canadensis, Gleditsia triaCanthos, and 

 Gymnocladus dioica (13} . The question at once arises whether these 

 species enrich the soil since they bear no nodules. According to 

 L. T. Leonard, of the Bureau of Plant Industry, as well as other 

 authorities, they probably do not. The synonymity of legumes and 

 nitrogen fixers is by no means certain ; possibly there are many more 

 legumes that do not fix nitrogen. 



EXPLANATION OF THE LIST 



SCOPE 



In the alphabetical list are considered all species of woody plants 

 known to grow in the continental United States and on the Cali- 

 fornia islands, with the following exceptions : 



1. Plants of the southern, subtropical portion of Florida. 



2. In general, species not woody throughout ; that is, woody only at the base. 



3. Species not native to the United States, unless definitely known to have 

 become naturalized to such an extent as to prove their ability to compete 

 with native species. 



4. The cacti, since, as Britton and Schafer (71) have said, stein structure 

 is such that no criterion other than size is available to determine which to 

 include as trees or shrubs and which to regard as herbs. 4 



5. Species of parasitic or partially parasitic habit, as Phoradendron, Razoum- 

 ofskya, Pyrularia, since growing or using them presents difficulties that in 

 effect exclude them from planting programs. 



6. Hybrid forms, except for occasional mention under the parent species. 



LATIN NAMES AND SYNONYMS 



The species making up the list of woody plants have been selected 

 from two sources: (1) Regional and local floras, and (2) monographs 

 of families or genera. But from whatever manual or monograph 

 the names were taken, a decision had to be made regarding the 

 proper name to use, since taxonomists are not always agreed on the 

 correct technical name. Furthermore, manuals published earlier in 

 the century, but still in common use, employ names not in accepted 



4 Cacti, especially the opuntias, have been used with considerable success in controlling 

 gully heads. The stem sections are simply dumped into the eroding gully, and on taking 

 root they form thickets that contribute materially to the control of the eroding soil. 

 Many species are very important as a source of food and water for desert animals. As 

 some 20 species of mammals and a like number of birds have been observed to eat cacti, 

 their importance to wildlife appears to be considerable. 



