ME. W. C. WOESDELL ON " TEAXSEUSION-TISSUE." 305 



Podocarpus Nageia, R. Br., Dammara, and Araucaria, among Conifers, a system of 

 parallel venation prevails, and here transfusion-tissue is markedly developed. 



The leaves of most Conifers are very narrow, and are traversed by a singh^ himdle, 

 which, in all cases, is provided with well-developed transfusion-tissue. 



Ginkgo differs widely from all other Conifers in having a dichotomizing system of 

 hundles traversing its large, fan-shaped leaf, and has transfusion-tissue present in 

 connection with its rather widely-separated hundles, though more feehly developed than 

 in most Conifers. 



In order to prepare the way for my subsequent remarks, I will first of all describe the 

 structure of the vascular bundle in the lamina of an ordinary foliage-leaf of a Cywid. 

 It is well known that the bundles of the leaf of Cycads have a structure peculiar to this 

 order and not found in any other living group of plants. Towards the dorsal (lower) 

 surface of the lamina is placed the phloem ; next comes the ordinary xylem, which is 

 formed by the cambium in a centrifugal * manner ; on the inner side of the secondary 

 wood there may or may not be a few elements of primary centrifugal wood, and then 

 follows the protoxylem, consisting of narrow, elongated, spirally- or reticulately-thickened 

 elements. Farther, beyond the protoxylem, i. e. between this tissue and the ventral 

 (upper) surface of the leaf, occurs another strand of xylem, primary in origin, and of 

 much greater development than that of the centrifugal wood; it is centripetal in 

 development, i. e. its elements are formed successively from the protoxylem towards the 

 ventral (upper) surface of the leaf; it is characteristic of the Cycadese. Typicjil 

 transfusion-tissue occurs at the side of the bundle, and this is seen to be in intimate 

 connection with the centripetal xylem (PL XXIII. fig. 2). In the petiole the structure 

 of the bundles is the same, though their orientation is different. In other Gymnosperms 

 and all Angiosperms this tissue is, so far as hitherto observed, absent from the vascular 

 bundle. No trace of any such tissue has previously been observed cither in the leaves 

 of the Coniferse or of the Gnetaceae. According to existing observations, the structure of 

 the bundle in these two latter groups of plants, as in aU Angiosperms, is endarch, in 

 contradistinction to that of the leaf of Cycadean plants, which is mesarch f. 



Original Observations. 



An investigation into the structure of the cotyledons of some seedlings of Ginkgo 

 biloba, Linn., grown in the Royal Gardens, Kew, revealed a most interesting and novel 

 structure. The cotyledons, as in the Cycadeae, are rudimentary, hypogeal structures. 



* The terms " centripetal " and " centrifugal," applied to the development of the parts of tho bundle, are used 

 with reference to the centre of the stem, in such a way that, in the case of the xylem starting from the first-formed 

 tracheae (protoxylem), elements formed successively near the phloem, i. e. towards the outside of the stem, are called 

 "centrifugal," and those formed successively in the direction away from the phloem, i.e. towards the centre of the 

 stem, are said to be developed " centripetally." The same terms ("centripetal '" and "centrifugal") are applied to 

 the development of the bundles in the leaf, whatever may be their orientation. 



t The term " mesarch" was first suggested by Solms-Laubach (Introd, to Fossil Bot., Engl. ed. p. 257) to define 

 the mode of development of the xylem of the bundle in Cycadean plants. In such plants the protoxylem occupies 

 a central position in the xylem of the mature bundle. This term was first definitely adopted by Williamson and Scott 

 in describing the structure of the stem-bundles in the fossil plants Lyginodendron and Ileterangium (Phil. Trans. 



