THE GENUS FRAXINUS IN NEW MEXICO AND ARIZONA. 



By Alfred Rehder. 



Received April 1'). l'.U7. 



\\'iiiLE collecting in Arizona and New Mexico for the Arnold 

 Arl)oretum in the summer of 1914 and 1916 I paid special attention 

 to the various species and forms of Fraxinus. There are at present 

 six species of Fraxinus known from that region. One of them, F. 

 LowcUii Sargent, is endemic to central Arizona; three, F. velutina 

 Torrey, F. Standleyi Rehder and F. ciispidata Torrey, are found 

 through a considerable part of the territory under consideration, 

 while F. anomala Torrey occurs only in northern Arizona where it 

 reaches the southeastern limit of its range, and the Mexican F. pa- 

 pulosa Lingelsheim just crosses the extreme southern border of New 

 Mexico. Though these six species are easily distinguished from 

 each other, as shown in the following key, they all exhibit a greater 

 or lesser degree of variability like most other American Ashes; particu- 

 larlv F. velutina is extremelv variable in its tomentum, in the size, 

 shape and texture of its leaflets, in the length of the petiolules and in 

 the shape and size of its fruits. The extreme forms might easily be 

 taken for distinct species, if it were not for the fact that they are closely 

 connected by intermediate forms and do not occupy separate geo- 

 graphical areas. This is particularly true of the glabrous and pubes- 

 cent forms which I found in almost every instance growing side by 

 side in the same locality. 



This study is based on material in the herbarium of the Arnold 

 Arboretum supplemented by a considerable number of specimens for 

 the loan of which I am indebted to those in charge of the National 

 Herbarium and of the Gray Herbarium; I also am under obligation 

 to Dr. N. L. Britton for the permission to examine and photograph 

 some of the types in the herbarium of the New York Botanic Garden. 

 All the species except F. papillosa known so far within our area only 

 from a single locality I have had the opportunity to observe and study 

 in the field. 



