No. 449-] NORTH AMERICAN CONIFERALES. 



337 



occupy nearly the entire surface of the wall within the limits of a 

 wood tracheid, thereby becoming few in number. In Sequoia 

 <Fig. 23) or Taxodium (Fig. 18) they are typically oval; in 

 Pinus cubensis or P. tceda (Fig. 24), they are variously lenticu- 

 lar, while in P. resinosa or P. koraiensis they are oval or oblong, 

 or even quadrangular. Such variations as a whole, are far more 





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numerous and sharply defined in Pinus than in any other genus 

 known. In all the investigated genera, the pit is bordered. 

 This finds either partial or complete exceptions in the genus 

 Pinus to the extent of 78.1 f of the species, in which the pits 

 are either simple throughout, or they exhibit a more or less 

 definite border in the summer wood only. That a border is a 

 characteristic feature of fossil representatives, is justified by 

 comparison with existing species, but it is not always recog- 

 nizable in consequence of the alterations of structure due to the 

 general process of petrifaction. Such obliteration not infre- 

 quently involves the pit orifice also. It is thus apparent that 

 such structures often fail in the determination of fossils. In 

 existing species the border is often so faintly defined as to be 

 difficult of recognition, and this is especially the case in rays of 

 a resinous character. In all such cases, however, the require- 

 ments of a correct diagnosis are fully met by the pit orifice. 

 The general law of development then, is such that all genera 

 except Pinus may be held to be characterized by bordered pits. 



