INTRODUCTION 



For several years Mr. E. W. Nelson, of the U. S. Department of Agriculture, in connection 

 with his biological explorations in Mexico, has collected specimens of the Pines of that country. 

 The very valuable material that he has brought together was gathered from various latitudes 

 and altitudes throughout Mexico and in parts of Guatemala and in localities remote from the 

 usual lines of travel. His collection, supplemented by that of his associate, Mr. E. A. Gold- 

 man and by that of Dr. J. N. Rose of the National Museum at Washington, and his assistants 

 was sent to me for identification and has formed the basis of this paper. In order to study the 

 trees as they grow naturally I have made four journeys to Mexico in company with Mr. C. G. 

 Pringle of the University of Vermont. 



A cursory examination of the specimens of Mexican Pines preserved in herbaria suggests 

 a large number of species; the varied altitudes and climates of Mexico point to a like con- 

 clusion. Each excursion among the living trees; however, increased my suspicion that the 

 numerous forms represent not many species, but several varieties of a few species. It was not 

 until this suspicion became conviction that a consistent systematic arrangement of the Mexican 

 Pines seemed possible to me. 



Two characters of Pinus may require explanation, i. The conelet, or cone of the first year 

 may be subterminal, lateral or pseudo-lateral on the branchlet. When the spring-shoot con- 

 sists of one internode only the conelet is necessarily subterminal ; when the spring-shoot con- 

 sists of two or more internodes, the conelet may be subterminal or lateral or both. When a 

 conelet is subterminal on the spring-shoot and a summer-shoot grows beyond it, the conelet 

 becomes pseudo-lateral. A pseudo-lateral conelet is easily recognized as the leaves of the 

 summer-shoot never attain normal length [Shaw, Bot. Gas. xliii, 205]. 2. At the end of 

 the growing season, buds enclosing staminate flowers are not sufficiently advanced in the Soft 

 Pines to be recognized, but in the Hard Pines they are usually easily distinguished by their 

 large size. The young male aments may either be concealed in the general outline of the bud, 

 in which case they can be found by removing the basal perulae, or they form about the nodes 

 of the bud characteristic enlargements which are constant for each species. 



In the bibliography of the species references are mostly confined to descriptions accompanied by illustrations ; 

 the exceptions are Parlatore, Endlicher and Hemsley. The last author gives no descriptions but his work 

 covers the whole of Mexico. 



Each specimen of the National Museum collections bears the collector's label and on each sheet is stamped 

 the Museum number. In referring to a specimen the collector's number is given followed by the Museum 

 number in brackets. 



