MR. W. C. T70ESDELL ON " TRANSFUSION-TISSUE." 303 



the latter these tend to disappear and bordered pits to be alone present. The development 

 of the tissue as a whole he considers to depend on the amount of transpiration to which 

 the leaf is subjected — this, again, depending on the habitat. 



As regards the origin of the transfusion-tissue, he approaches nearer to the truth than 

 any other author These are his words : — " Wherever, in the mature condition, the 

 transfusion-tissue is found surrounding the phloem or the xylem, or both, its otHgiu lies 

 in the xylem, as it completely passes over into the latter.^'' This is, nevertheless, a vague 

 statement, and not very much value can be attached to it. 



The paper, as a whole, is extremely suggestive, and, I think, has been rather over- 

 looked by subsequent writers. 



A dissertation by Vetters *, entitled * Die Blattstiele der Cycadeen,' gives a most 

 minute and thorough description of tlie structure of the petioles, and the course of the 

 bundles in the pinnae, of all Cycadean genera. There is a careful and accurate account 

 of the "accessory transfusion-tissue" in Cycas\ and he also notifies the occurrence, 

 though under varying forms, of the normal transfusion-tissue on the side of the bundles 

 of the pinna. 



The most accurate and thorough investigation of transfusion-tissue, as it occurs in the 

 group Abietiuese, which has hitherto been made is that by Daguillon t- He is the first 

 to define the exact tissue of the leaf in which, in this group, the tracheidcs occur, 

 viz. the pericycle. He is careful to notice the various positions with regard to the bundle 

 which the tracheides of the transfusion-tissue occupy respectively in the cotyledon and 

 foliage-leaf. He finds that in the cotyledon of Finns sylvestris, Linn., they occur opposite 

 the protoxylem ; in the foliage-leaf, on the contrary, chiefly round the phloem ; an 

 intermediate stage is presented in the '* primordial " % leaf, where the pericyclic scleren- 

 chyma, which he holds to be homologous with the transfusion-tissue, extends round both 

 xylem and phloem, being most abundant round the latter. His figures illustrating these 

 points are the clearest and finest ever published. 



A great authority on the subject, Van Tieghem §, describes the occurrence and the 

 varying positions of transfusion-tissue in the leaves of Conifers. He recapitulates 

 the statements of Frank, Mohl, and De Bary as to its origin, concluding that the 

 views of these authors are inadmissible, and that the transfusion-tissue, from his own 

 observation, belongs morphologically to the peridesm (pericycle) of the bundle. 



Lignier H, the latest author dealing with the subject, discusses the homology of the 

 transfusion- tissue in the pinna of Cycas, and finds that the tracheides of the " accessory 

 transfusion-tissue" which traverse the mesophyll to the margin of the leaf merge 



♦ Inaugural Dissert., Leipzig, 1884. 



t " Recherches sur les Feuilles des Coniferes," Revue generale de Bot. tome ii. 1890. 



X The " primordial " leaves are the first^formed leaves of the seedling, which succeed the cotyledons, and occur 

 singly and scattered around the stem ; they possess a flattened lamina with a single bundle, thus differing consider- 

 ably from the later-formed " needles." 



§ " Sur les Tubes cribles extraliberiens et les Vaisseaux cxtraligneux (' needles')," Journ. de Bot., 1891. 



II " La ly^ervation teniopteridee des Folioles de Cycas et le Tissu de transfusion, " Bulletin de la Soc. Linn, de 

 Normandie, ser. IV. tome vi. fasc. 1. 



2y 2 



