590 
Eames. — On the Occtirrence of 
ment, as shown by the same authors, is likewise centrifugal ; the protoxylem 
of the internodal bundles is continuous with the innermost elements of this 
xylem-band. In Equisetum , too, there has been no question raised, so far 
as is known to the writer, as to the centrifugal development of this supra- 
nodal wood. Certainly its innermost elements are protoxylem ; many 
radial sections show this condition clearly. 
Although it is generally agreed that the supranodal wood and the 
carinal strand in Equisetum are homologous with the similar tissues in 
Calamites, there has been some difference of opinion in regard to the 
interpretation of the remaining parts of the bundle. 
The two metaxylem-groups, in the internode distinct from the clearly 
endarch protoxylem, have the appearance of being exarch, for the tracheides 
constituting them are often smallest at the outer extremity, and increase in 
size with some constancy inwards. Mr. Gwynne-Vaughan 1 emphasizes 
this fact, and bases thereon his suggestion * that the lateral xylem strands 
in the vascular bundles of the existing Equisetums may perhaps be taken 
to represent the last remnants of a primitive central mass, and that this 
would be entirely in agreement with their apparently centripetal develop- 
ment, and in particular with their cauline course \ Poirault 2 also considers 
the development of the metaxylem centripetal, and Lignier 3 regards it as at 
least partially so. The first-mentioned author at the same time doubts the 
integral nature of the Equisetal fibro-vascular bundle, stating that of 
the three groups of tracheides in each internodal bundle the carinal alone 
leaves the node, passing out as the leaf-trace, and ‘ the two lateral strands 
join on to the xylem of the nodal ring, and in certain species may be traced 
as externally projecting ridges over the nodal xylem into the internode 
above \ The fact that the metaxylem-groups, so definitely separated from 
the protoxylem, develop much later than the latter may be said also to 
point to the composite character of this bundle. 
The study of the vascular system of Equisetum , both nodal and inter- 
nodal, developing and mature, and of that of the reproductive axis, together 
with a comparison with the stele of the Calamites, has led the present writer 
to the conclusion that the bundle of the Horse-tails is essentially centrifugal 
in development and, without doubt, simple, or integral, in nature. 
Let us grant for the moment the centripetal development of the 
flanking metaxylem-groups in the bundle. The supra-nodal wood is 
centrifugal. And into or over this, fusing therewith, pass these strands. 
Adopting the first supposition, we find centripetal wood passing directly, 
1 Gwynne-Vaughan, D. T. : Remarks upon the Nature of the Stele in Equisetum. Ann. Bot., 
vol. xv, p. 776 ; 1901. 
2 Poirault, G. : Developpement des Tissus dans les organes vegetatifs des Cryptogames vas- 
culaires. Mem. de l’Acad. Imp. des Sc. de Saint-Petersbourg, 7 e serie, t. xxxvii, p. 13 ; 1890. 
3 Lignier, O. : Equisetales et Sphenophyllales ; leur origine filicineenne commune. Bull. Soc. 
Linn, de Normandie, 5 e serie, vol. vii, p. 114, note. Caen, 1903. 
