316 MR. W. C. WOESDELL ON " TRANSFUSION-TISSUE." 



of such an origin for a transfusion-tissue, and a conspicuous instance thereof is found in 

 the stem of Casuarina *, where trans fusion- tissue, consisting of tracheides with simple pits 

 and lignified walls, is formed directly from the parenchyma-cells immediately surrounding 

 the cortical bundles, and, of course, here quite independently of any centripetal xylem, a 

 tisssue which these bundles never possessed. But what I maintain is that in Gymno- 

 spermous plants the trans fusion- tissue is nothing more or less than a direct extension of 

 the centripetal xylem. This could scarcely be decided upon from an examination of the 

 foliage-leaves of either Cycads or Conifers, but was conclusively revealed by an investiga- 

 tion of the cotyledons of these plants. The transitions in size and other characters 

 which take place, both in a ventral and in a lateral direction, between the elements 

 of the centripetal xylem directly opposite the protoxylem and those of the peripheral 

 transfusion-tissue, left no doubt as to the truth of the above inference. Had the 

 tracheides of the transfusion-tissue been formed independently from the pericyclic or 

 mesophyll-cells around the bundle, I see no reason why there should have been found 

 such easy and delicate transitions in size, &c., between these and the centripetal xylem. 

 It is true that there is a difference in size, both of diameter and length, between tlie 

 cells of the mesophyll and those of the perieycle; but this, though it might possibly 

 account for the transitions in size and shape above mentioned, yet would not account 

 for the transitions in the characters of the cell- wall which are frequently met Avith, 

 nor, indeed, for the fact that in the cotyledonary bundles the transfusion -tissue is 

 better developed on the ventral than on the lateral side of the bundle. In tlie 

 cotyledonary bundle of Ginkgo, as indicating the place of its origin, the traclieides of 

 the transfusion-tissue are frequently seen to be mingled witli the ordinary tracheides 

 of the centripetal xylem, and, in one case, an element of the former, witli reticulations 

 on its transverse wall, was seen jammed in between two elements of the latter tissue. 

 Again, if the transfusion-tissue were a distinct and independent formation, it would, I 

 imagine, not be so conspicuous in the cotyledons, e. g. of Ginkgo, as is in reality the 

 case. In these organs it can have but little function to perform. In the foliage-leaf, 

 on the contrary, of Ginkgo, where the importance of its function is obvious, it is but 

 very sparingly present as compared with its occurrence in the cotyledon of this plant. 

 The raison d'etre, then, of its having an origin independent of the centripetal xylem 

 in this case falls to the ground, while my own view, that the transfusion-tissue is a 

 modification of the centripetal xylem, is clearly supported by these facts. 



That in those cases, of which there are several, where the transfusion-tissue is in direct 

 connection with the centrifugal xylem, there are seen to be transitions in size between 

 the elements of the two tissues, does not affect my position when it can be shown that 

 this is a purely secondary result, for in the cotyledonary bundles of Ginkgo and Cycas, 

 and in several other plants which I have examined, the transfusion-tissue is always to be 

 seen extending out from the region on the ventral side of the protoxylem. The transition 

 just mentioned is due, doubtless, to the difference in size between the parenchyma-cells 

 immediately adjoining the bundle, viz. those of the perieycle, and those, belonging to the 

 ground-tissue of the leaf, lying farther away. 



On the whole, then, I think the facts justify my position, viz. that the normal 



* " On the Comparative Anatomy of the Casuarinac," etc., Ann. Bot. vol. viii. (1894). 



