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THE AMERICAN NA TURALIST. [Vol. XXXVI II. 



tions, as also upon the radial, terminal, upper and lower walls of 

 the ray tracheids may be held to represent a feature somewhat 

 distinct from their presence on the walls of the wood tracheids, 

 and their consideration properly belongs to a discussion of the 

 medullary ray as a whole ; but it may be observed that they 

 constitute a characteristic feature in the structure of the ray 

 elements in the great majority of the Coniferales. 



The occurrence of bordered pits on the walls — especially the 

 radial walls — of the wood tracheids in the Ginkgoales and Coni- 

 ferales, is 'much too familiar a fact to call for special discussion 

 at this time, but reference may be made to the additional fact 

 that their characteristic structure is such as to permit of their 

 use for the general purpose of tracing possible lines of descent 

 through such extinct types as Cordaites and the Cycadifilices. 

 It is true that similar bordered pits originating in modifications 

 of spiral structures, are to be met with, often in great numbers, 

 in the higher angiosperms, but in such cases the associated 

 structures permit of a clear and definite differentiation of all 

 such woods from the Coniferales. 



Radial wrt//^.— The characteristic situation of the bordered 

 pits is on the radial walls where, as was shown many years since 

 by De Bary (9, p. 160), "the pits of contiguous tracheids always 

 correspond to one another in such a way that on each limiting 

 surface, all the cavities of the pits of one fit exactly over those 

 of the other. The plano-convex cavities are thus applied to one 

 another in pairs so as to form the lens-shaped pit cavities " as seen 

 in tangential section. But on surfaces abutting on elements of 

 another order, e. g., parenchyma cells, the bordered pits of the 

 tracheids correspond to non-bordered pits, or they are opposite 

 an unpitted wall. Four typical variations of the bordered pits 

 may be recognized : — ( i ) the multiseriate, when they are dis- 

 posed in any number of rows more than two, (2) the 2-seriate, 

 (3) the uni-seriate with occasional pairs of pits, and (4) the 

 strictly uni-seriate. The general sequence thus presented will 

 be found to be in direct accord with the evolution of higher types 

 of structure and organization. 



The most primitive type of gymnosperm presenting a multi- 

 seriate arrangement, is the genus Cordaites. Among eleven 



