Observations on the Centripetal and Centrifugal 
Xylems in the Petioles of Cycads . 1 
BY 
M. J. LE GOC, Ph.D. (Rome), B.A. (Cantab.), 
The Botany School , University of Cambridge. 
With Plate XI and one Figure in the Text. 
T HE Cycads possess a number of characters which may be considered 
as sign-posts pointing out possible paths of evolution. A noteworthy 
example is the peculiar structure of the foliar bundle with its double xylem 
arrangement, one portion centripetal, the other centrifugal (PI. XI, Figs. 7-10). 
The significance of this unusual type of structure was first appreciated 
and carefully investigated by Mettenius. 2 Most of the observers who 
followed him have interpreted the facts in relation to some special line of 
evolution from Palaeozoic plants. Hence it is that while facts are generally 
agreed upon, the interpretations differ according to the theories the facts 
are supposed to prove, corroborate, or favour. 
We consequently find a class of writers considering the foliar bundle of 
Cycads as strictly mesarch, agreeing closely with the Lyginopteris (Lygino- 
dendron ), to which genus the Cycads are usually considered to be related. 3 
Another school opposes this view on the ground that it is based on 
a mere appearance : the bundle is apparently and not strictly mesarch. It 
is due to an inversion of the primary normal xylem. c Pour certains auteurs, 
et ce sont les plus nombreux,’ says Bertrand, ‘le bois centrip&te n’est autre 
chose que le bois primaire normal retourn6. >4 Some authors, e. g. Chodat, 
would derive the centrifugal xylem from the centripetal xylem. ‘ Le bois 
centrifuge serait, d’apres Chodat, du metaxyleme detache lateralement.’ 5 
Bertrand’s own opinion, in his collaboration with Renault, is summarized 
in these words : ‘ Un faisceau unipolaire diploxyle est done un faisceau 
unipolaire normal dans la partie superieure duquel le bois primaire s’est 
1 A summary of this paper was given before Section K at the British Association, Birmingham, 
2 G. Mettenius (’61). 
3 W. C. Williamson and D. H. Scott (’96). 
4 C. Eg. Bertrand et B. Renault (’86). 
5 A. Sprecher (’07), p. 34. Cf. also R. Chodat, Sur quelques fossiles. 
[Annals of Botany, Vol. XXVIII. No. CIX. January, 1914-] 
