600 E antes. — -On the Occurrence of 
disappeared. Equisetum shows signs of relationship to these ancient plants 
in the possession of centripetal wood in regions recognized as conservative 
of ancestral characters. The axial bundles of the strobilus are weakly 
mesarch ; those of the sporophylls, strongly so. This condition in the 
sporophyll-trace is not due to the concentric arrangement of the sporangia 
upon the scutes, for the subdivisions of the trace, one of which passes 
to each sporangium, are themselves equally mesarch throughout their 
course. The traces of these fertile leaves are likewise concentric. The 
traces of the vegetative leaves show well-developed centripetal wood, in 
some species throughout their course. In no case does the passage of 
a foliar trace from the stele leave a gap. 
Equisetum presents additional and very strong evidence of the con- 
servatism of the leaf-trace, thus emphasizing its value in phylogeny. 
Centripetal wood is now known to exist in all the large groups of 
Vascular Cryptogams. In all those in which it is well developed it is con- 
tinuous with the protoxylem ; where it no longer appears at all, as in the 
higher plants, or where only vestiges are found, as in Equisetum , the proto- 
xylem usually becomes continuous with the centrifugal xylem. The 
formation of xylem adaxially from the protoxylem is, therefore, a Crypto- 
gamic character. Moreover, bundles in which both centripetal and centri- 
fugal xylem are present cannot be said to be characteristic of any one group 
of the Vascular Cryptogams, as, for example, of the Ferns, nor can such 
bundles in higher plants be of other phylogenetic value than as indicating 
general Cryptogamic affinities. 
This work has been carried on in the Phanerogamic Laboratories 
of Harvard University, and to Dr. E. C. Jeffrey the writer is much in- 
debted, both for abundant material and for advice and guidance throughout 
the investigation. 
DESCRIPTION OF PLATE XLV. 
Illustrating Mr. Eames’s paper on Centripetal Xylem in Equisetum . 
Fig. r. Lycopodium , sp. ; transverse section of part of the central cylinder during lignifi cation 
of the xylem. x 180. 
Fig. 2. Equisetum hyemale , var. robustum ; transverse section of a developing internodal 
bundle, x 500. 
Fig* 3 * The same; radial section through the origin of the vegetative leaf-trace, showing 
connexion with both protoxylem and metaxylem. x 100. 
Fig. 4. The same ; radial section through vegetative leaf=trace passing through cortex, showing 
mesarch structure, x 500. 
