ME. W. C. WOESDELL OX " TRaNSFUSION-TISSUE." 300 



of C]/cas is also an independent formation, as how otherwise can one account for the 

 intercalation of the normal transfusion-tissue between the midrib and this "bundle- 

 remnant" ? This " accessory transfusion-tissue," as I propose calling it, is, as Lignicr 

 rightly says, an extension of the transfusion-tissue proper, the latter being, as it seems to 

 me, an older formation, acquired by the ancestors of the plant soon after the lateral 

 veins were dropped, and the former, as it were, an afterthouglit to ensure a more perfect 

 supply of water to the distant cells of the broad lamina of the leaf ; there may also be 

 much truth in Zimmermann's view, that this tissue serves the mechanical purpose of 

 strengthening the leaf, in the same way as would a system of lateral veins. 



In all other Cycads a dicliotomous system of venation prevails without any midrib 

 whatever. This dichotomy is suiSciently evident and conspicuous in the broad pinna? of 

 Bowenia and Zamia, and may also be seen pretty clearly at places in the lamina of 

 Ceratozamia . In those genera with narrower pinnge, such as Dioon and 3Iacrozamia, 

 the dichotomy is quite obscured and hidden from superficial observation owing to the 

 fact that, as Vetters and Lignier showed, and as I have myself since verified, the branching 

 of the bundles takes place low down in the very base of the pinna, or in the extreme 

 edge of the rachis ; as, subsequently, no further branching occurs higher up in the pinna, 

 the result is a series of parallel veins of equal strength traversing the mesophyll. In all 

 these other genera normal transfusion-tissue is present at the sides of the bundle, as in 

 Cycas. But the elements composing it are always inconspicuous and scarcely recogniz- 

 able as transfusion-tissue such as it is seen in Cycas. They are often clearly seen in 

 connection with the centripetal xylem extending along the sides of the bundle. It is 

 only occasionally that these elements exhibit bordered pits or reticulations on their 

 transverse walls (PI. XXV. fig. 12). Vetters, while recognizing the presence of these 

 tracheides, refused to regard them as equivalent to the transfusion-ti^acheides of Cycas ; 

 but I have no doubt that he was wrong in this respect. 



In the order Conifcrse there obtains a far greater diversity in tlie conformation of the 

 foliage-leaf than in the order we have just been considering. Ginkgo stands entirely 

 alone both as regards the shape and the venation of its leaf, and clearly indicates a much 

 less modified and more primitive type of structure than is found in any other genus of 

 the order. The broad fan-shaped lamina and dichotomous venation sufficiently prove 

 this to be the case. 



As I have already dwelt upon the structure of the vascular bundle in the foliar organs 

 of Ginkgo, I will now pass on to its nearest allies among the Coniferae, \iz. the 

 Taxinese. In these the leaves are much reduced and in no wise resemble those of 

 Ginkgo. 



Cephalotaxus drupacea, Sieb. & Zucc. — In the cotyledon of this plant the bundles, 

 which are the least varying parts of the leaf, present a structure strikingly similar 

 to what obtains in Ginkgo. Transfusion-tissue is scarcely to be distinguished, and is 

 only represented by some tracheides at the corner of the centrifugal xylem. The 

 centripetal xylem, on the other hand, as seen in transverse section, is very well developed, 

 forming a nearly continuous band along the ventral side of the bundle, and consisting of 



SECOND SERIES.— BOTANY, VOL. V. 2 Z 



