Anatomy of the Cone and Fertile Stem of Equisetum . 671 
less the middle of a xylem-strand may be termed median 
internodal strands remain isolated the median 
position of the traces is very obvious, but when 
the bundles are united into bands or into a ring 
at the nodes, as they often are in E. arvense , the 
median position of the traces is often not so clear 
(PI. LXIV, Fig. 1). Such formation of nodal bands 
giving off median traces also occurs in E. palustre , 
though it is not so marked a feature of this species 
(e. g. the band from which the third, fourth, fifth, 
and sixth traces of the first whorl depart in 
Cone A, and the band giving rise to the fifth, 
sixth, seventh, and eighth traces of the fifth whorl 
of the same cone). In reality we find three sorts 
of structure above median traces. Firstly, there 
is the form in which the xylem of the axis is most 
developed ; here there is no mesh, and a relatively 
wide band of xylem persists through the internode ; 
this type, never common, occurs in Equisetum 
arvense , and apparently in the upper part of some 
cones of E. palustre. Secondly, a parenchymatous 
mesh may appear above a median trace ; this is 
by far the most widely spread form in E. arvense ; 
it is very common too in E. palustre , especially 
in cones with well-developed xylem. The third 
form of structure is associated with a reduction 
of axial xylem ; here the trace departs from the 
middle of a relatively narrow strand ; this often 
continues its isolated course upwards, and then of 
course involves the persistence of meshes on either 
side of it ; above such a trace no mesh is formed. 
When the meshes on both sides of a strand persist 
through several internodes the latter pursues an 
isolated course through several nodes (e. g. the 
strand arising between the sixth and seventh traces 
of the fifth whorl of Cone A of E. palustre'). 
These conditions are obviously associated with 
poor development of the xylem. A variation in 
the behaviour of the strands in the internode 
above the departure of a median trace of this type 
occurs when one of its neighbours gives off a median 
trace of the other type, above which trace there 
appears a fresh parenchymatous mesh ; or in other 
When the 
Text-fig. 3. Longitudinal 
reconstruction of the xylem of 
Cone A of E. palustre. Axial 
xylem black ; leaf-traces and 
parenchyma white. Magnifi- 
cation circa 9. 
