MEDULLARY RAYS 87 



in radial series, or in the marginal cells and low rays upwards 

 of six per tracheid." In Taxodium distichum the pits are round, 

 conspicuously bordered, and large, with a very narrowly lenticu- 

 lar and diagonal orifice, which is often as long as the outer limits 

 of the pit. But in the analytical key it will be observed that 

 this genus is naturally brought into close relations with Sequoia, 

 which is also distinguished by large bordered pits. The ulti- 

 mate differentiation then rests upon the fact that in the latter 

 the pits are oval, the border often narrow, sometimes obscure, 

 while the oblong or lenticular, usually rather broad, orifice is 

 generally parallel with the cell axis. As a final illustration, the 

 four pits of Pinus monophylla, or the one to five throughout, 

 finally reduced to one to two in the summer wood of P. Bal- 

 fouriana, point with much definiteness to these particular species, 

 while among the hard pines the occurrence of large oval or 

 squarish pits, one or rarely two per tracheid, segregates a group of 

 four species. Detailed as these features are, they are not acci- 

 dental, but of such constancy as to admit of no hesitation in 

 accepting the conclusions to which they point. 



The length of the ray cell is subject to considerable variation 

 not only within the limits of an individual but also between one 

 species and another. Our studies, however, do not permit the 

 formulation of a law applicable to specific differentiations, even 

 if such a law does exist, which present evidence leads us to doubt ; 

 but details of length, in terms of spring tracheids, have been 

 incorporated in all the diagnoses, since they are often very 

 suggestive and thus may assist in the ultimate recognition of 

 the species. 



The form of the cell is of more evident value, although too 

 much stress must not be laid upon it. The cell is either straight, 

 as in Juniperus, Libocedrus, or Picea (fig. 23), or it becomes 

 fusiform through contraction of the extremities, as in Cupressus, 

 Sequoia, Taxodium, etc. (fig. 21). As a well-defined differential 

 character its value is only one degree higher than the length of 

 the cell, and for the same reason it has been introduced into the 

 diagnoses as a controlling factor of secondary importance. 



