256 SIGILLARIEAE. 



The peculiar structure of the trace-bundle of Cycadeae has been well 

 explained by Mettenius 1 , who was the first to notice the distinction between 

 the centripetal and centrifugal portions of the xylem, between which 

 lies the initial strand, and the terminology of the French authors may 

 therefore be traced directly to him. But at the same time the terms which 

 they employ are unfortunately chosen, since they may mislead the reader 

 into regarding the two parts of the bundle as anatomical members essentially 

 distinct from and entirely independent of one another. Mettenius was 

 perfectly aware that this is not the case ; he is careful to show how the 

 initial strand, so placed in the inner angle of the leaf-trace in the segment 

 of the stem that there is only ' bois centrifuge ' present, gradually moves 

 towards the outside while the bundle is still young, till it has reached the 

 central position characteristic of the leaf-segment. It is true that two 

 woody portions now appear on the single transverse section, but they are in 

 no respect essentially different from one another; they are connected laterally 

 below, and together they form the one xylem-strand of the bundle, in which 

 the displacement of the initial group has simply given rise to an unimportant 

 alteration. It appears to me therefore that the French anatomists, van 

 Tieghem and his school, have not rightly understood Mettenius, when they 

 oppose the two parts to one another as ' bois centripete ' and ' bois centri- 

 fuge,' and regard them as distinctly different things. Thus we find van 

 Tieghem 2 saying unreservedly : ' This second portion of the wood (centri- 

 fugal) corresponds to the normal wood of the cauline bundle ; it is the fan 

 formed by the centripetal wood which is the superadded portion.' This is 

 not correct ; it is the two parts together which correspond to the ' bois 

 normal,' and nothing beside is added to it, only the initial point of the 

 ultimate development has suffered displacement. And this fundamental 

 error appears in other authors in a more advanced and more pronounced form. 

 Thus Bertrand and Renault 3 have written quite recently: 'The cauline 

 bundle preserves its structure when it enters the leaf, only its primary wood 

 is reduced and a new tissue, the centripetal wood, is intercalated between 

 its pole and its anterior face. The centripetal wood of a unipolar diploxy- 

 lous bundle is therefore not the homologue of the displaced primary wood of 

 the normal unipolar bundles. The tissue is a relic of a former organisation.' 

 There is scarcely a word in this sentence to which I am not obliged to 

 take exception. Mettenius' figures themselves teach the exact contrary, 

 but I have also satisfied myself perfectly by personal examination of various 

 young individuals of the group of Cycadeae that the actual state of things 

 is as I have here portrayed it. Had Mettenius adopted a different ter- 



1 Mettenius, G., Beitrage zur Anatomic der Cycadeen in Abh. d. kgl. sachs. Ges. d. Wiss. Bd. vn 

 (Math. Natw. Cl. vol. v), Leipzig, 1861. 2 van Tieghem, Traite de hot. 1884, p. 812. 



3 Bertrand and Renault (1). 



