ON MESARCH STRUCTURE IN LYCOPODIUM" 
EDMUND W. SINNOTT 
(WITH PLATE xX) 
Halftone 
The position of the protoxylem in relation to the later-formed 
elements of the wood has been the subject of careful investigation in 
the various groups of vascular plants, both fossil and living, and has 
often been brought forward as evidence of the relationship of one 
group to another. 
In his description of the anatomy of a specimen of Sigillaria, in 
1839, BRONGNIART (1) showed the central cylinder to be formed of a 
ring of vascular bundles, each composed of an inner, primary part, 
and an outer, secondary part, the latter characterized by the radial 
arrangement of its elements. The small spiral elements lie on the 
outer edge of the primary wood, which was all developed in a cen- 
tripetal direction, while the secondary wood grew centrifugally. The 
protoxylem is thus surrounded by later-formed elements. 
In his classic work on the anatomy of the cycads, METTENIUS (2), 
in 1861, described the structure of the vascular bundle in the petiole 
and blade of certain living cycads. He showed that in the leaf-trac¢, 
as it leaves the cylinder of the stem, the protoxylem is all on the inside 
of the later-formed, mostly secondary, wood, but that as the bundle 
ascends into the petiole, the later-formed wood gradually bends 
around inwardly and incloses the spiral elements. Finally the ce™ 
trifugal wood, which is probably all secondary, is greatly reduced, 
and separated from the protoxylem by parenchyma, while the ce™ 
tripetal wood forms the bulk of the bundle, and bears at its outer 
most point the cluster of spiral tracheids. METTENIUS (2, P- 582) 
noticed the resemblance between this bundle and one of the ste™ 
bundles of Sigillaria, and remarks that the much greater development 
of centrifugal secondary wood in the latter is due to the fact that 
belongs to the vascular system of the stem. ‘ 
RENAULT (3), however, was the first to really suggest the aflim'y 
ii sen wise from the Phanerogamic Laboratories of Harvard University, 
no. 10, 
Botanical Gazette, vol. 48] ; (138 
