Notes. 
769 
opposite the protoxylem of very many of the bundles of the central 
cylinder in the form of very small, lignified elements equalling in size the 
innermost primary tracheides of the bundles. In manyplaces these small 
elements are seen to pass gradually over into larger and larger lignified 
cells which, in transverse section, are seen to be scattered on all sides, 
around the bundle and far out into the pith, as reticulate elements of the 
same shape as the adjacent parenchyma- cells. As seen in longitudinal 
section, the smallest of these presumably centripetal tracheides, viz. 
those nearest the protoxylem of the cylinder, are very faintly and 
indistinctly outlined as compared with the tracheides farther out in 
the pith, which fact points to their representing a tissue on the way 
towards becoming effete. It is true all these tracheides bear but little 
resemblance to the centripetal xylem known to us in other plants, 
for they possess simple pits, and thus in appearance suggest the idea 
of ordinary sclerotic cells ; but the typical sclerotic cells of the plant 
occur side by side with them, and curious, slightly-lignified groups of 
fibres are also scattered throughout pith and cortex. Moreover, I can 
detect no difference between the character of the pitting of these 
elements and that of the secondary tracheides of the bundle itself; 
and further, the gradual transition existing between the much 
elongated, narrow elements adjoining the protoxylem of the cylinder, 
and the short, reticulate, parenchyma-like elements of the more distant 
region, is precisely the same phenomenon met with in the foliar bundles 
of Cycads and Coniferae which exhibit centripetal xylem, and is 
unlike anything we should expect to find in a system of sclerides. 
In fact, I conclude that we may have here, as in the similar instance 
of the stem of Cephalotaxus ( Podocarpus ) koraiana observed by 
Rothert l , and the peduncle of Stangeria , &c. 2 , a case in which the 
centripetal xylem of the bundles of the axis, instead of becoming, 
as in the great majority of cases, entirely suppressed and extinct, 
is retained either as a mechanical tissue or as a mechanical and con- 
ducting tissue combined ; but the pitting of whose elements, in the 
case before us, has become modified, and which has become extended 
throughout the ground-tissue, as in the case of the centripetal xylem 
1 Rothert, W., ‘Ueber parenchymatische Tracheiden und Harzgange im Mark 
von Cephalotaxus- Arten.’ Berichte d. deutsch. bot. Gesell., Bd. xvii, 1899. 
2 Scott, D. H., ‘ The Anatomical characters presented by the Peduncle of 
Cycadaceae.’ Ann. Bot.. vol. xi, 1897, Plate XX, Fig. 2, and Plate XXI, 
Figs. 8 and 9. 
