Xylems in the Petioles of Cycads. 191 
Bertrand bases his assertion on the fact that the centrifugal xylem 
comes out of the stem undistorted ; but this is hardly an argument, as the 
centrifugal xylem might well be inverted higher up in the petiole, as it 
apparently is. His other argument is based on the idea that the proto- 
xylem attached to the centrifugal xylem gradually disappears. There is 
much to be said for such a view, only that it is not easy to show that this 
protoxylem is disconnected and different from the protoxylem which 
accompanies the centripetal xylem . 1 
It would seem that at least the centripetal xylem has been reduced at 
one of its extremities ; the few blind xylem elements scattered at the base 
of the petiole indicate that they formerly extended lower down, perhaps 
even into the stem ; £ it has only survived as long as it has because of the 
conservatism of the foliar bundles ; ’ 2 while the centrifugal xylem might 
possibly have been secondary all along its course, approaching some types 
of Palaeozoic fossils, e. g. Poroxylon. 
The terminology used by different observers reflects accurately the 
theories these observers have in mind. According to Dr. Scott, the term 
mesarch ‘ implies that the development begins in the middle of the strand 
of wood \ 8 It is difficult to find such a bundle in the petiole of Cycads 
where the protoxylem is connected (a) either with a xylem which is entirely 
centrifugal, or (b) with a xylem which is centripetal, in which case the 
protoxylem and the centrifugal xylem of secondary origin are usually 
disconnected. 
The French school, on the contrary, constantly employ the word 
‘ diploxylic ’ — 4 diploxyle ’ — because ‘ they oppose the two parts to one 
another as “ bois centripete” and “ bois centrifuge ”, and regard them as 
distinctly different things ’ (Solms-Laubach ). 4 Then Solms-Laubach adds 
that if the word ' mesarch ’ had been invented before the French authors 
published their results, ‘ we might perhaps have been spared this miscon- 
ception/ This remark is tantamount to saying that a hypothesis may be 
derived from a coined word rather than a definition or word be made, 
created if necessary, to fit in with a theory. 
W ound-struc tures. 
Do the structures artificially induced throw any light on this problem ? 
There is now a tendency to consider such structures as primitive. In this 
view the anatomical modifications brought about in the petiole of Cycas 
by cuts would be most instructive. The bundles much influenced looked 
concentric ; it would then follow, according to this theory, that the foliar 
bundles were originally concentric, that they have lost this character and 
passed through all the stages met with in passing away from a wound. 
1 C. Eg. Bertrand et Renault (’86). 2 D. H. Scott, New Phyt., 1902, p. 29. 
3 W. C. Williamson and D. H. Scott (’96). 4 Solms-Laubach (’91), pp. 256-7. 
