MEDULLARY RAYS 85 



either partial or complete exceptions in the genus Pinus to the 

 extent of 78. i per cent of the species, in which the pits are either 

 simple throughout or else they exhibit a more or less definite bor- 

 der 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 recognizable in consequence 

 of the alterations of structure due to the general process of 

 petrifaction. Such obliteration not infrequently involves the pit 

 orifice also. It is thus apparent that such structures often fail 

 in the determination of fossils. In existing species the border 



FIG. 26. PINUS SEROTINA. Medullary ray showing (i) the ray tracheids with 

 dentate walls ; (2) the structure of the parenchyma cells ; (3) tracheids con- 

 terminous with parenchyma cells, x 280 



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 requirements 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. Their strong tendency to obliter- 

 ation in that genus is found to coincide with the more marked 

 development of ray tracheids, which undoubtedly assume more 

 completely the original functions of the parenchyma cells, these 

 latter in consequence suffering constant structural reduction, as 

 in the hard pines. 



