ANATOMY AND PII YL()(JKN Y OK THE C0NIFERALE8. 



31 



by Thomas ('66), there may be but a single strand throughout. This author is of the opin- 

 ion that the single bundlo is more primitive, and cites Ginkgo as ;m example in which the 

 increased development of the leaf leads to the appearance of a double foliar trace. With 

 our present knowledge of the more ancient and especially of the fossil Gymnosperms, tins 

 interpretation caunot he accepted, for we have now every reason to believe that the 

 ancestors of the Coniferales had much larger foliar organs, much more richly supplied 

 with strauds of librovascular tissue, than their descendants. The progression is con- 

 sequently in the opposite direction, i. v., the more modern Gymnosperms have suffered 

 reduction both in leaf surface and in the librovascular tissue constituting the leaf traces. 

 The simplification of the leaf traces into a single strand has apparently been from above 

 downwards. In further support of this view may be cited the observation made by 

 Scott ('97) in the case of the reduced strands occurring in the peduncular portion of the 

 cone in certain species of Cycads. Here paired strands coming off from the central 

 cylinder of the cone, run a greater or less distance upward into the cortex and then 

 become fused together. Dr. Scott considers it probable that these double strands repre- 

 sent abortive leaves. It' is interesting in connection with the present argument that 

 there should be the same fusion of originally separate leaf traces at the apex in the case 

 of reduction. 



Other indications of primitive affinity, less important it is true, because less wide 

 spread and constant, are the occurrence of a chambered pith in Abies and in certain 

 species of Pinus, and in the case of the leaf trace of the former genus, the complete 

 enclosure of the phloem 'of the laminar bundle of the leaf in certain instances, by a 

 cordon of transfusion tissue. The first of these features was very prevalent in the 

 Cordaites although not universally present (Scott, :02), while the second has also been 

 recently described in a Cordaitean leaf (Stopes, :03). 



If the arguments advanced in the foregoing paragraphs are sound, we may assume, 

 because they present many features of structure which unite them both with the 

 older Gymnosperms on the one hand, and by an obvious process of reduction, with the 

 Cupressineae, etc., on the other hand, that the Abietineae are a very ancient family or 

 order of the Coniferales. This conclusion is favored by what is known at the present time 

 of their geological occurrence, and will probably receive fuller confirmation from this 

 source when the geological record is more complete than it is at the present time. 



The affinity of the Abietineae with other cohorts of Gymnosjjerms. — The discussion 

 of the relationship of the Abietineae with other great groups of Gymnosperms has been 

 largely anticipated in the paragraphs of the last section. Both Coulter and Chamberlain 

 (:01) and Scott (:00) among recent writers on the subject of the phylogeny of the 

 Gymnosperms have derived the Coniferales as a whole from a Cordaitean stock or plexus. 



