Scott.— On Pedum le of Cycadaceae. 405 
in the vascular bundles of the sporophylls, that no importance 
can be attached to this difference h 
Centripetal xylem was found in all the male peduncles 
examined— five in number— which approached maturity. It 
is, however, developed late, as was proved by the comparison 
of a series of transverse sections, extending from base to top 
of a young peduncle, with corresponding series from more 
advanced specimens. The centrifugal wood of the young 
peduncle had attained a thickness of three or four elements, 
but in no part was any centripetal wood as yet differentiated. 
In another peduncle, with centrifugal wood five or six 
elements thick, the centripetal elements were already fairly 
numerous ; they only attain their full number, however, in 
nearly mature peduncles, when the centrifugal xylem has 
from seven to twelve tracheides in each radial series. This 
late development of the centripetal wood accounts for the 
shortness of its tracheides, for at the time when they develop 
the tissue is too mature to allow of any great degree of 
sliding growth. For the same reason, spiral elements are 
rare, on the centripetal side, for as growth in length is usually 
over when these tracheides are differentiated, no provision 
for extensibility is needed. In one case, however, a very 
characteristic spiral tracheide, with a fairly close coil, lay 
between the disorganized protoxylem and the scalariform 
elements of the centripetal wood (Fig. 7 x). Here then there 
was a gradual transition from the primitive to the definitive 
tracheides, in both directions, but as a rule the centripetal 
xylem develops too late for such a transition to be traced. 
The more internal tracheides of the centripetal wood are 
sometimes met with while still in course of development, as is 
best shown in radial sections. 
This belated appearance of the centripetal wood in the 
1 In the pinnae of Bowenia spectabilis , Hook., and sometimes in the petiole 
also, I find the protoxylem in contact with both parts of the wood equally ; the 
same is the case in the pinnae of Ceratozamia mexicana, and of Macrozamia 
Denisonii , F. Muell. and M. heteromera , as well as in the petiole of Siangeria. 
In the rachis of Dioon edide , Lind, the spiral elements often lie midway between 
the two xylems. Cf. Mettenius, loc. cit. p. 580. 
