THE 



AMERICAN NATURALIST. 



Vol. XXXVIII. May, igo^. So. 449. 



THE ANATOMY OF THE NORTH AMERICAN 

 CONIFERALES TOGETHER WITH CERTAIN 

 EXOTIC SPECIES FROM JAPAN AND 

 AUSTRALASIA. 



(Continued from page 27s). 

 D. P. PENHALLOW. 



The Medullary Rays. 



The medullary ray, in the various details of its structure as 

 presented radially and tangentially, comprises some of the 

 most important features for diagnostic and taxonomic purposes. 

 While it presents numerous variations, these are, in the main, of 

 such a nature as to give them very positive \aluc for both ,<;cn- 

 eric and specific differentiations. Primaril\- the medullary ra\ s 

 are to be regarded as a residue of the ori^qnal fundanu-ntal 

 structure which has been left over in the genesis of the primary 

 stele, but they are capable of reproduction or extension under 

 the influence of the cambium in the course of secondary groutli. 

 In all such cases, however, they are typically composed of the 

 same elements which are necessarily parenchymatous. Devia- 

 tions from this structure may arise through the introduction of 

 other elements, but such alterations always arise in a manner 



