MR. AV. C. WORSDELL ON " TEANSFUSION-TISSUE." 315 



Thmihergii, Pari., which I examined, I could, however, find no trace of transfusion-tissue 

 anywhere. 



IHnus Pinaster, Soland. — In a transverse section of the foliage-leaf tlie transfusion- 

 tissue occurs all round the bundle. But besides these elements, there were observed, 

 immediately on the ventral side of tlie protoxylem, a group of thin-walled, irrci^ularly- 

 shaped, rather small-celled elements with lignified walls. In a lonj^itudinal section th(^ 

 parenchymatous elements abutting on the protoxylem appeared very much elongatetl, 

 with either oblique or horizontal walls, and had undergone a great amount of sliding- 

 growth. Among them were found trachcides with very small, but distinct, bordered 

 pits ; but in many of these thin-walled trachcides, which are of the same diameter and 

 length as those of the protoxylem, the bordered pits are almost obliterated, and, in a 

 great many, are entirely gone. It seems to me that these trachcides represent the last 

 lingering remnant of the centripetal xylem, which is gradually becoming resolved into 

 the parenchymatous elements above mentioned. Outside one of tliesc trachcides just 

 described were seen two tracbeides of the transfusion-tissue, of which it is to bo noted 

 that the innermost is much more elongated and narrower than the other one, this latter 

 representing the more normal form of these trachcides (PI. XXVI. fig. 21). 



The following plants afford additional instances in which I have observed centripetal 

 xylem in the bundle of the leaf: — Cupressus torulosa, D. Don; Cunninghamia sinensis, 

 R. Br. ; Saxegothea conspicua, Lindl. ; Prumnopitys elegans, Phil. ; Torreya califoniica, 

 Torr. ; and T. nucifera, Sieb. & Zucc. 



General Results. 



Collating all these facts with the structure, as we have long known it, of the vascular 

 bundle of the leaves of Conifers and Cycads generally, it appears to me that there is a 

 clue to be found as to the origin of the transfusion-tissue in these plants. This tissue, as 

 seen in the cotyledonary bundles of Cycas and Ginkgo, is clearly an extension towards 

 the sides of the bundle of the centripetal xylem of the latter. As this tissue began to 

 extend itself irregularly both in a ventral and in a lateral direction away from the 

 protoxylem, the trachcides forming its lateral extension were finally fixed, and, occurring 

 as they did at the sides of the bimdle, bordering on the surrounding mesophyll of the 

 leaf, were eventually made use of as a permanent and useful auxiliary conducting- 

 system, this latter being found necessary on account of the inadequate distribution of 

 the bundles in the mesophyll of the leaf. In the case of Conifers, where the centrifugal 

 xylem has considerably increased in amount, the centripetal xylem, as such, tends to 

 disappear, but persists in a modified form when of use as an auxiliary conducting-system. 

 An objection to this view of the origin of the tissue in question has been raised on the 

 ground that trachcides, such as those of the transfusion-tissue, may be formed aiiyulh k . 

 at any time, and from any tissue of the plant. The transfusion-tracheides, it is urged, 

 may have been developed quite independently of the centripetal xylem, and have become 

 directly continuous with its lateral region, from the fact that this part of the xylem is the 

 most convenient for the attachment of the transfusion tissue. I quite admit the possibility 



