4 H Scott. — On Peduncle of Cycadaceae . 
It appears then that the Cycadean type of vascular bundles 
— one of the most ancient of anatomical characters — was not 
originally a mere peculiarity of the leaf, but rather represents 
the vestige of a primitive organization, which was once 
common to the stem also 1 . 
In recent Cycads the stem has all but lost this character ; 
it survives, so far as we know, only in certain peduncles, 
organs which appear to have been less modified than the 
vegetative stem. 
The loss of the centripetal wood was in all probability 
correlated with the increasing development and earlier 
appearance of secondary tissues. The earlier the stage 
at which the formation of wood by the cambium set in, 
the more would the primary centripetal xylem become 
superfluous. We can form an idea of the progress of this 
change by the comparison of some of the fossil forms. In 
Heterangium the secondary tissue-formation set in late, and 
was, as a rule, comparatively small in amount. In H. Grievii 
especially, the great bulk of the wood is primary, and nearly 
all the primary part is centripetal, occupying the whole 
interior of the stele. In Lyginodendron Otdhamium the 
central part of the wood has disappeared, and is replaced 
by a large pith, around which the bundles are ranged. In 
each bundle the centripetal portion of the wood is well 
developed, but secondary growth sets in early, and the 
secondary wood soon far exceeds the primary in amount 2 . 
In Lyginodendron robustum , Seward, no primary centri- 
petal wood has been certainly demonstrated as yet ; if 
present, it must have borne a very small proportion to the 
secondary tissues. In Cycadoxylon the former tissue seems 
to have all but disappeared, much as in recent Cycads 3 . 
1 This view has already been stated in the joint paper on Lyginodendron and 
Heterangium by the late Dr. Williamson and myself. See p. 768, 1. c. 
2 For Heterangium and Lyginode 7 idron see Williamson and Scott, loc. cit., and 
the earlier papers by Williamson there cited; also Seward, Palaeobotany and 
Evolution, Science Progress, October, 1896. 
3 See Seward, A Contribution to our knowledge of Lyginodendron , Annals of 
Botany, March, 1897. The author constantly uses the term ‘centripetal wood’ 
