ME. W. C. WORSDELL ON " TRANSFUSlON-TISSrE." 317 



transfusion-tissue (not the " accessory transfusion-tissue "), as at present known in the 

 foliar organs of Gymnosperms, is not an outlying tissue of no morphological value with 

 regard to the vascular bundle to which it is attached, but that it has a distinct and 

 definite origin in this bundle, viz, from the centripetal xylem ; that it was the successive, 

 itnlimited, centripetal development of the tracheides of this latter tissue which, generations 

 back, afforded, as it were, the first start, which has eventually culminated, in more modern 

 plants, in the characteristic transfusion-tissue at the side of, or in various positions around, 

 the vascular bundle. The albuminous cells or " pbloem transfusion-tissue," called by 

 Strasburger " CJebergangszellen," appear to arise, on the contrary, quite independently, 

 from the parenchyma-cells immediately flanking the phloem. Though evidently similar 

 in their function of conducting substances between the cells of the surrounding mesophyll 

 and the bundle, the two kinds of transfusion-tissue have, nevertheless, a very different 

 origin, the one springing from the vascular tissues of the bundle itself, the other from 

 the parenchyma lying outside these vascular tissues. The former, the xylem transfusion- 

 tissue, is not, as some have supposed, a distinctly new tissue derived from the parenchyma 

 of the ground-tissue of the leaf, or even of the pericyclic cells ; nor is it again, as others 

 have imagined, the equivalent of a lateral vein or branch of the bundle. The conclusion 

 at which I have arrived is that this tissue is a direct derivative of the centripetal xylem 

 which normally occurred as an important part of the vascular bundle in the ancestors of 

 the ConifersB (for which fossil plants afford ample evidence), and which still occurs in 

 full development, along with the transfusion-tissue, in modern Cycads. But as, in the 

 course of time, the centripetal xylem of the bundle, in the case of the Coniferae, 

 disappeared, as having become a useless tissue, the origin of the transfusion-tissue, 

 which has persisted as a highly useful portion of the bundle, though still perceptible in 

 Cycads, has, in the Coniferae, become almost completely obscured. 



Summary. 



The chief points to be gathered from the investigation into the subject of this paper 

 are the following : — 



1. The universal presence of transfusion-tissue in the leaves of Gymnosperms whose 



venation is of a reduced or rudimentary character, and w^here the complex 

 reticulate or dichotomous venation of the Ferns or tbe higher Angiosperms is 

 lacking. 



2. The conspicuous development of centripetal xylem in the vascular bundles of the 



foliage-leaves of all modern Cycads, without exception, where it constitutes the 

 most important part of the xylem of the bundle. This is a point which has long 

 been familiar to botanists. 



3. The discovery, for the first time, of a mesarch structure of the vascular bundle in tlie 



leaves of Coniferse. In the cotyledonary bundles of Ginkgo biloba, Linn., where 

 this was first observed, the centripetal xylem forms the chief part of the woody 

 strands and is very conspicuously developed. In the bundles of the cotyledons 

 and mature leaves of all other Coniferae which were examined the centripetal 

 xylem is extremely reduced, this reduction having, in most cases, corresponded with 

 a greater development of the centrifugal xylem. It is most obviously present in 



SECOND SERIES. — BOTANY, VOL. V. 3 A 



