304 ME. W. C. WOKSDELL ON " TEANSFUSION-TISSTJE." 



insensibly into the tracheides of the normal transfusion-tissue immediately adjoining the 

 bundle, and that there is thus no real distinction to be made between the two. He 

 considers the transfusion-tissue in the leaf of Cycas and of all Gymnosperms, not as a 

 later structure than the bundle, but as the remnant of a dichotomously-branched venation 

 which was a common feature in their ancestors, and is still found in the allied genus 

 Stangeria and in some Perns. 



In another important and interesting paper * he establishes the fact that the vascular 

 bundle-system in the pinnae of all Cycads is really dichotomous, though, owing to the 

 fact that the branching of the bundles sometimes takes place in the very base of the 

 pinna or on the outermost edge of the rachis, this feature, though often obvious even 

 to superficial observation, has not been recognized as generally characteristic of the 

 order. 



Strasburger f describes fully the position and structure of the transfusion-tissue in the 

 leaves of Gymnosperms, especially those of Coniferse and Cycadese ; his book is certainly 

 the best compendium for a general and accurate account of the anatomy of Gymno- 

 spermous leaves. 



General Considerations. 



Transfusion-tissue consists of short tracheides, parenchymatous in shape, and with 

 reticulate or other thickenings or bordered pits, on all the walls (PI. XXIII. fig. 1). Its 

 usual position is at the sides of the bundle in the pericyclic region ; but it may frequently 

 extend, as in Araucaria and Libocedrus, in an arc round the protoxylem, or it may, as 

 in Ticea and Pinus, completely encircle the bundle. Its evident function, as has been 

 rightly inferred by various authors, is to supplement, in the leaves of these plants, what 

 must be considered a rudimentary conducting-system. In a number of Conifers the 

 leaf is traversed by but a single bundle (in some Abietinese by two closely-contiguous 

 bundles) ; in others, and in most Cycads, by a number of widely-separated, parallel 

 bundles. In all these plants there is an entire absence of the complex, reticulate system 

 of conducting-tissue, such as is met with in Dicotyledonous plants, and which must be 

 considered the highest type of venation in the vegetable kingdom. In order to 

 compensate, therefore, for the lack of an efficient conducting-system in the leaf, recourse 

 has been had to the development of these peculiar tracheides (often accompanied by 

 bast-cells of similar shape), now known as " transfusion-tissue." In Cycas and many 

 species of Podocarpus, in which the broad pinnae or leaves are traversed by a single 

 bundle, in addition to the normal transfusion-tissue, a new and accessory system has been 

 developed, running from the bundle to the margin of the leaf. This, however, as will 

 be shown later on, is a purely secondary modification of the mesophyll- cells, and bears 

 only a functional relation to the normal transfusion-tissue, having therewith no homology 

 whatever. In the pinna of Stangeria a dichotomizing system of closely-placed veins 

 springs from the large central midrib. In the pinna? of all otiier Cycads, and in 



* " La Nervation des Cycadees est dichotomique," Assoc. Franc^-aise pour I'Avancement des Sciences. Congres de 

 Caen, 1894. 



t ' Ueber den Bau and die Yerrichtangen der Leitungsbahnen,' Jena, 1891. 



