MEDULLARY RAYS 107 



in which they attain their most complete development, admits 

 of only one interpretation. The fact that they are exclusively 

 features of the Coniferae emphasizes their inferior value for de- 

 termining the derivation of that group, while it points to their 

 superior importance as a factor in the sequence of the various 

 coniferous genera. They occur sporadically in Thuya (i), Cu- 

 pressus (2), Juniperus (i), and Abies (i). They are prominent 

 features of Tsuga, Pseudotsuga, Larix, Picea, and Pinus. Their 

 invariable absence from Sequoia would appear to suggest that 

 this genus is more primitive than Thuya, but there are other 

 reasons which serve to suggest the opposite relation. Apart 

 from this exception, it will be seen that in accordance with the 

 relations exhibited in the table of anatomical data (Appendix A) 

 the genera enumerated form a continuous series, commencing 

 with those showing sporadic tracheids and ending with those in 

 which such structures attain their highest expression. From 

 this we are justified in the conclusion that the rare occurrence 

 of tracheids in Thuya, etc., is to be interpreted as the first evi- 

 dence of a tendency in development which is only fully realized 

 at a later period, and this appears to be justified by a closer 

 examination of the last five genera in this respect, since it is 

 found that in them the tracheids not only show a progressive 

 numerical development but their structure likewise becomes 

 more complicated in direct relation to the evolution of higher 

 types of genera and species. We must therefore look upon the 

 tracheids with their thin, simple walls as the primitive form, 

 while those with the strongest reticulations are of the highest 

 type, the two being united by a transitional form characterized 

 by the presence of simple teeth. The evidence at hand does 

 not appear to justify the idea that the various genera have been 

 segregated into small groups representing side lines of develop- 

 ment, but it rather favors the thought that each genus is in 

 itself a complete short line of descent, and that among these a 

 prominent parallelism has arisen in the tendency toward the 

 development of tracheids, a tendency which has been carried 

 to completion in the case of only five of the series, and in such 



