190 Le Goc . — Observations on the Centripetal and Centrifugal 
normal centrifugal xylem of the stele and the girdles ; the centripetal 
xylem is an independent tissue, probably the remnant of an ancient and more 
developed structure — a conclusion supported or suggested by the structure 
A 
Text-fig. Longitudinal section of apex of stem and of petiole, showing relation between 
centripetal and centrifugal xylem. cpx , centripetal xylem ; px , protoxylem ; cb, cambium ; cfx, 
centrifugal xylem; ph , phloem, a, younger petiole without centrifugal xylem except at base; 
b, older petiole with centrifugal xylem throughout, but not in connexion with the protoxylem 
except at the base before the centripetal xylem appears. 
of several Palaeozoic types . 1 It is worthy of remark that the first centri- 
petal xylem elements to be found at the base are often scattered and 
unconnected with the centrifugal xylem. The text-figure shows the relation 
between the two xylems. 
1 D. H. Scott, Ann. Bot., 1912, p. 1014. Cf. also New Phyt., 1902, pp. 25-30. 
