306 ME. W. C. WOESDELL ON " TEANSFUSION-TISSUE." 



They are thick and fleshy in consistence, bearing little, if any, resemblance to leaves, and 

 are closely adpressed by their ventral faces, though not completely connate, as in the 

 Cycadese. The tip of each is slightly notched. The cotyledons are traversed each by 

 two bundles which in shape, as seen in transverse section, are curved in the form of an 

 arc of a circle. There is a great development of the phloem of the bundle. The 

 centrifugal part of the xylem is but very feebly developed ; some of its elements can be 

 seen to be secondary and cut off by the cambium, some of the smaller and innermost are 

 probably primary. To the inside of the latter lies the small group of protoxylem. On 

 the ventral side of the protoxylem, however, and directly opposite the latter, there are 

 yet other tracheides present with densely spiral thickenings, which, by their position and 

 relative development, I determined to be none other than the equivalent of the centri- 

 petal xylem as it occurs in the bundles of Cycadean leaves. These tracheides are very 

 much scattered (PI. XXIII. fig. 3). A row of three, however, more compactly grouped, 

 show clearly the nature of the bundle, to which the term " mesarch," as descriptive of the 

 bundle of Cycadean leaves, may fitly be applied. The remaining tracheides occupy 

 various positions. In proportion as they recede farther from the protoxylem towards 

 either side of the bundle, they attain a greater diameter and are provided with reticulate 

 bars of thickening on their transverse walls ; they moreover assume a perfectly angular 

 outline as seen in transverse section. In fact, they present very much the appearance 

 of the tracheides composing the transfusion-tissue in the leaves of Coniferae. One or two 

 of these tracheides which most completely resemble transfusion-tissue are situated quite 

 on the side of the bundle and bordering on the phloem (fig. 3). In longitudinal 

 secti(m of the bundle, a most evident and interesting transition is seen between the 

 tracheides nearest the protoxylem, which are elongated and narrow in shape, much 

 resembling the elements of the latter, and those which are farthest removed from the 

 protoxylem, which are short and stout, thus entirely different from the former tracheides, 

 and in every way similar to the elements of the transfusion-tissue of Coniferous leaves 

 (fig. 4). In the bundles of the lamina of the foliage-leaf of Ginkgo, tracheides, to the 

 number of one or two, are usually present on the ventral side of the protoxylem, but, 

 compared with the cotyledon, the centripetally-formed xylem is here very much reduced. 

 There is typical transfusion-tissue at the sides of the bundle, consisting of wide tracheides 

 with conspicuous reticulate thickenings and bordered pits (PI. XXIII. fig. 5). In the 

 bundles of the petiole there is a great development of the secondary centrifugal xylem ; 

 consequently, the number of elements of the centripetal xylem is exceedingly reduced, the 

 tracheides being very inconspicuous and scattered (PI. XXIV. fig. 6). Fig. 7 shows a good 



Roy. Soc. vol. 186 (189.5) B, pp. 703-779). The term «' endarch," kiudly suggested to me by Dr. 1). H. Scott, is 

 employed here for the first time to define the structure of a bundle in which, as in the stems of the great majority 

 of Phanerogams, the whole of the later-formed xylem is centrifugal, i. e. is developed towards the phloem, the 

 protoxylem always occupying the innermost portion of the bundle. Van Tieghem (Traite de Bot. pp. 763 & 764) 

 uses the two terms perixylic and centroxylic ; the former would include the terms " mesarch " and " exarch," the 

 latter would be the equivalent of " endarch." 



