232 MORPHOLOGY OF GYMNOSPERMS 



remarked by Van Tieghem (35), and is taken to mean that the most 

 primitive type of xylem has persisted unchanged in the vascular 

 system of the root. 



The spiral vessels of the primary xylem, through various inter- 

 mediate elements, grade into the characteristic tracheids of the second- 

 ary wood. These tracheids are arranged radially and have bordered 

 pits that are usually restricted to the radial walls. The pits occur 

 in a single linear series, except among the Araucarineae, in which 

 they are multiseriate, that is, in two or more alternating series and so 

 crowded that the pits are deformed by mutual pressure. This type 

 of tracheid with multiseriate .bordered pits is so characteristic, that 

 when it was discovered entering into the structure of paleozoic woods, 

 they were named Araucarioxylon. As has been stated, much of the 

 wood referred to Araucarioxylon has been found to belong to Cordai- 

 tales, so that it is no longer possible to distinguish the wood of Cor- 

 daitales and Araucarineae; but this situation has left the impression 

 that the araucarian type of tracheid is more ancient than the other 

 type (96). Attempts have been made to distinguish the various 

 tribes and even genera of conifers by their tracheids, in the hope of 

 applying the distinctions in the determination of fossil woods, but they 

 have not been successful. Bailey (153) has recently illustrated this 

 in the case of Picea, Larix, and Pseudotsuga, showing that such charac- 

 ters as wood parenchyma, spiral thickenings, and resin canals, often 

 relied upon in determining the genera of fossil Abietineae, are too 

 sporadic or variable to be used with any certainty. 



The structure of the medullary rays has been found to furnish 

 important characters for comparative study. The varying details 

 of this structure are foreign to our purpose; but in general two types 

 of ray are recognized: (i) the linear ray, consisting of a single series 

 of cells in tangential section; and (2) the fusiform ray, which is broad 

 enough to contain a resin canal (96). The medullary rays of the 

 living Abietineae are so complex, with their parenchyma cells, ray 

 tracheids, and elaborate system of resin canals, that they have been 

 used as evidence of a highly specialized and modem group. A recent 

 study of the ray tracheids of Cunninghamia by Jeffrey (143), 

 however, has indicated the possibility that the simpler rays may 

 have been derived from more complex ones. The marginal ray 



