Anatomy of the Cone and Fertile Stem of Equisetum. 679 
cases, and a few occur also in Cone C, the strand that does not give off a trace 
is never wide, but it is no narrower than many trace-bearing strands. Such 
excess of strands over traces must not be confused with an apparent increase 
in the number of strands due to the forking of a strand above a median 
trace occurring a little before the fusion of one or both of its branches with 
a neighbouring strand or branch of a strand, or to the decurrence of a 
parenchymatous mesh below and to one side of what would otherwise be 
a median trace. This apparent increase of the strands at a node may be 
exaggerated by the fact that the sporangiophores of a whorl are not all 
inserted at exactly the same level. Thus in one transverse section just above 
a node of Cone C, there were twenty-five strands and parenchymatous 
meshes, though the number of sporangiophores in the whorls above and 
below was but twenty. Such apparent increase of strands is, however, 
strictly localized to the neighbourhood of the nodes, and the supernumerary 
strands, if traced upwards or downwards for any distance, soon cease to be in- 
dependent strands, and may thus be distinguished from those much rarer dis- 
tinct and separate strands that pass through a node without giving off a trace. 
Alternation and Superposition of Whorls of the Cone. 
In Equisetum generally, the sporangiophores appear externally to 
alternate with remarkable regularity ; in the young state before the inter- 
nodes have elongated the shields are closely imbricated. But in internal 
anatomy there is throughout the three species examined no such regular 
alternation in the insertion of the vascular traces of successive whorls. It 
is obvious that when a parenchymatous mesh extends through more than 
two internodes, all the members of a whorl cannot be accurately superposed 
to those of the second whorl in a downward direction as they are when 
succeeding whorls alternate regularly. In fact, the greater or less regularity 
of alternation of traces of successive whorls depends chiefly on the regularity 
with which successive meshes are closed at the nodes. In cones of E. arvense , 
especially in those that, like Cones A and B, have their xylem well developed, 
meshes persisting through a node are in a small minority, and the traces 
are much more often alternate with those of the node below. Even here the 
alternation is liable to be disturbed by the occasional occurrence of meshes 
of the second and third orders ; by a change in the number of members of 
a whorl; by the fact that sometimes, though very rarely, a mesh may 
originate not vertically above a trace, but at a point above and to one side 
of it ; and by the fact that when two sporangiophores become partially 
concrescent, their traces tend to become slightly approximated at their point 
of origin. When a trace has divided prematurely into two closely approxi- 
mated bundles the mesh is superposed to both, and its position is one of the 
means of distinguishing a case of premature division of a trace (cf. Text-fig. 2, 
traces 6 and 7 of the first whorl). When by a close approximation of 
