SEVENTH LECTURE. 



A CLASSIFICATION OF THE NORTH AMERICAN 



TAXACEAE AND CONIFERAE ON THE BASIS 



OF THE STEM STRUCTURE. 



D. P. PEXHALLOW. 



Heretofore botanists have been so accustomed to rely 

 wholly upon characters derived from the external parts of 

 woody plants as a basis of classification that, with few excep- 

 tions, it appears to have escaped serious consideration that 

 those external characters which permit us to differentiate 

 families, genera, and species must also be represented by 

 corresponding variations in the internal structure, and, that 

 these, also, may constitute important and reliable data as a 

 basis of classification. 



It is now forty-six years since Goppert, in his well-known 

 work,i endeavored to establish the relations of certain fossil 

 coniferae to existing species. In the prosecution of this work 

 a number of living species from Europe and America were 

 studied critically with respect to the details of structure as 

 represented in the woody parts of the stem. Numerous figures 

 illustrating the minute internal anatomy accompany the diag- 

 noses of species. So accurate are they that it is possible to 

 determine species from them with as much facility and accuracy 

 as if freshly drawn. The characters are, in fact, precisely 

 those which recent investigations have shown to be of generic 

 and specific value. Although at that time no special attempt 

 was made to formulate a classification on this basis, yet the 

 results detailed were destined to have such an important bearing 



1 Foss. Conif., Leiden, 1850. 



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