142 ECONOMIC WOODS OF THE UNITED STATES 



BING-POEOUS AND DIFFUSE-POROUS WOODS 



There are 36 indigenous genera, exclusive of shrubs and vines, 

 with ring-porous woods, at least in part, and 4 that have become 

 thoroughly naturalized in the United States. These 40 genera 

 are representatives of 20 families of which only four, each con- 

 sisting of a single genus, are exclusively ring-porous. Sixteen of 

 these genera supply wood of more or less economic importance. 

 In the case of Quercus the live oaks are mostly diffuse-porous, 

 while one species of Hicoria and one or two of Ulmus are rather 

 intermediate, at least in certain instances. Some species of Prosopis 

 are diffuse-porous and some other genera, e.g., Leitneria and 

 Ptelea, produce woods which require rather close observation to 

 note their ring-porous nature. 



There are 35 families whose indigenous representatives are 

 exclusively diffuse-porous. Eleven of these families include 15 

 genera of economic woods. Of a total of 147 indigenous dicotyle- 

 donous woods, 113 or nearly 80 per cent are diffuse-porous. Inso- 

 far as the economic woods are concerned, however, the division is 

 about equal. 



