MR. W. C. WORSDELL ON "TEANSFUSION-TISSUE." 307 



example of such a tracheide in longitudinal section. In this part of the leaf the 

 transfusion-tissue is also but little developed. 



A study of the structure of the bundles in the cotyledons of Cycas revolufa, Thunb., 

 revealed something very similar to what was found in those of GlnJcgo biloha, Linn. There 

 are a number of bundles in each cotyledon, arranged somewhat in the form of a triangle. 

 Here, as in Ginkgo, there was a relatively small development of the centrifugal, hut a 

 great development, on the contrary, of the centripetal xylem. Here also, in the latter 

 tissue, could be distinctly seen a transition between the elements nearest the protoxylem, 

 which were elongated and of small diameter, and other elements which were short, of 

 great diameter, and possessing conspicuous bordered pits on their transverse walls, which 

 occurred scattered in the ground-tissue, often at a considerable distance from the bundle 

 and chiefly in a lateral direction. It was interesting to see how some of these tracheides 

 extended round towards the phloem, exactly as occurs in the bundles of the mature 

 foliage-leaf (figs. 8 & 9). In fig. 10 is represented a longitudinal section of a similar 

 bundle. 



I will now proceed to consider the structure of the vascular bundles of the foliar 

 organs of Gymnosperms generally. This may be deemed, perhaps, a rather useless task, 

 as botanists may assert that the structure of the leaves of these plants is already well 

 knowm. Nevertheless, I hope, before the close of this paper, to exhibit a feature of 

 considerable importance which has hitherto apparently been unsuspected by all 

 investigators. 



I will begin with the order Cycadeae. These plants are known to be intermediate in 

 their general structure between Conifers on the one hand and the great Pern-group on 

 the other. In the structure and general appearance of the leaf Stangeria certainly 

 approaches nearest to the Perns. The broad, elongated pinnae of the leaf are traversed 

 by a single thick midrib ; from this a dense venation of dichotomizing bundles radiates 

 out on either side to the margin of the leaf. The vascular bundle, as in all Cycads, 

 possesses centripetal, as well as centrifugal, xylem. Transfusion - tissue, though 

 inconspicuous and of small development, is present at the sides of tlie bundles. It 

 is conceivable that Cycas, in the structure of its leaf, may be a nearer relative than 

 any other Cycad to Stangeria. The pinnae, while retaining the single midrib in 

 much reduced form, have become, relatively to their length, very much narrower, 

 as a consequence of which the lateral veins, typical of Stangeria, have completely 

 disappeared, leaving the pinnae provided with but a single vascular strand, the remnant 

 of the bundles traversing the much thicker midrib of the pinna of Stangeria. Of the 

 tissues composing the bundle in the midrib of Cycas, the centrifugal xylem is, as in all 

 Cycads, but feebly developed, and is secondarily formed by the cambium. The centri- 

 petal xylem, on the contrary, is extremely well developed, and consists of tracheides 

 increasing in diameter as they extend towards the ventral side. The transfusion-tissue, 

 forming a dense group of tracheides at the sides of the bundle, is that typical of all 

 Cycads. In the intermediate pericyclic region there extends in an oblique direction a 

 group of tracheides connecting the main part of the centripetal xylem with the 



