ANATOMY AM) PHYLOGENY OF THE CONTFERALES. 



II 



the cortex of the axis its double character can generally he clearly distinguished. Ber- 

 tram! ('74) describes a double trace as characteristic of the genus Abies in general, and to 

 this statement I can subscribe. In figure 23, plate 3, is represented a foliar trace of A. 

 magnified in transverse section at a height of two millimeters in the free part of the leaf. 

 The striking feature of this figure is the fact that the transfusion tissue, connected with 

 the bundle of the leaf in this as in most other Conifers, is unusually abundant, and instead 

 of forming a scattered reticulum about the fibrovascular strand, interspersed with living 

 parenchyma cells as is the rule, constitutes a dense and continuous ring of tracheidal 

 tissue, completely surrounding the phloem. There is a tendency for it to become inter- 

 rupted opposite the cambium on either side of the bundle by parenchyma cells. The con- 

 dition of the transfusion tissue found in this case is to be compared with that recently 

 described by Miss Stopes (:03) in the leaf of a Cordaite, probably the Cordaites princi- 

 palis of Renault. In his description of the bundles of this leaf, Renault interprets a layer 

 of tracheary tissue outside the characteristic centripetal wood, found so commonly in the 

 leaves of the more ancient Gymnosperms, as centrifugal so called phanerogamous wood, 

 similar to that occurring, for example, in the bundles of the petiole of the Cycads. 

 Miss Stopes has pointed out that this so called centrifugal wood is separated from the 

 centripetal xylem by a layer of irregularly disposed, thin walled tissue comparable in 

 every way to primary phloem. Moreover the tracheary tissue in question is attached to 

 the flanks of the centripetal wood, and its elements, arranged in the form of a double 

 sheath, correspond to the transfusion tissue of the Conifers and other Gymnosperms. 



In this connection Miss Stopes calls attention to the researches of Worsdell on trans- 

 fusion tissue in the Gymnosperms. Worsdell ('97) reaches the conclusion that the trans- 

 fusion tissue, " which occurs almost universally in the leaves of Gymnospermous plants as 

 an auxiliary conducting system, has been phylogenetically derived from the centripetally 

 formed xylem of the vascular bundle." In the example in figure 23, plate 3, the trans- 

 fusion tissue extends well round towards the back of the flanks of the bundle, and there 

 are even some elements formed in a centripetal direction. Miss Stopes draws the conclu- 

 sion that the sheath described by her in Cordaites principalis is not centrifugal wood, but 

 "a primitive stage in the development of transfusion tissue from centripetal xylem." 

 The condition of affairs shown in our figure 23 seems to be strong evidence for the truth 

 of this hypothesis, and it supplies a transition from the condition ordinarily found in the 

 Conifers to that sometimes found in Cordaites as described by Miss Stopes. At a higher 

 level in the leaf the continuity of the transfusion tissue on the outside of the phloem is 

 generally lost, and we have resulting a condition in which there are merely massive 

 wings of transfusion tissue on the flanks of the bundle, a state of affairs which is perma- 

 nently present in Sequoia, Cunninghamia, etc. A good figure of this arrangement of the 



