THE BASIS OF THE STEM STRUCTURE. 179 



completed. Within recent date, however, all the various 

 species and varieties of the Taxaceae and Coniferae north of the 

 Mexican boundary have been brought together. This fact, 

 joined to the preeminent position occupied by these plants with 

 respect to their economic importance and palaeontological rela- 

 tions, led to their being regarded as subjects best suited to 

 immediate investigation. The present results, therefore, which 

 deal with the gymnosperms only, may be regarded as the first 

 of a series of similar investigations on the classification of the 

 North American woods as a whole. 



Incidentally to the present work, a number of foreign species 

 have been studied, but it has been thought advisable to defer 

 their consideration until opportunity offers for an exhaustive 

 treatment of exotic species. 



The whole number of species and varieties included in our 

 present studies is eighty-nine, representing fourteen genera. 



The investigations, the results of which are now given, had 

 their origin in 1880. They possess no claim to originality 

 beyond the methods of working out the details, but the results 

 now reached amply confirm the conclusions of Solereder, as 

 already referred to, with respect to the stability of family, ge- 

 neric and specific characters, and, although more extended con- 

 firmation is needed before a final statement can be made, it 

 seems possible that varieties may also be recognizable. 



The history of the Coniferae abundantly shows that great 

 difficulty has always been experienced, not only in defining the 

 specific limitations, but in establishing the relations between 

 the various genera. Of this there are several notable examples. 



Of the closely related representatives of the genus Picea 

 occurring in eastern America, Link distinguished three species, 

 which he designated as P. alba, P. nigra, and P. rubra. Later, 

 botanists on this side of the Atlantic very generally refused to 

 recognize the validity of the latter, which was held to be only a 

 form of P. nigra, and this has been the ruling practice, with one 

 or two exceptions, up to the present time. In 1879, however, 

 Englemann admitted rubra as a variety of nigra^ and this was 

 subsequently admitted as valid by some of our leading author- 



1 Bot. Works, 351 ; Card. Chron., N.S., XI, March 15, 1879. 



