Anatomy of the Cone and Fertile Stem of Equisetum. 239 
is very clear in the great majority of cases. In estimating the super¬ 
position and alternation from the longitudinal reconstructions, allowance must 
be made for the convergence or divergence of the imaginary lines of. super¬ 
position in accordance with the decrease or increase in width of the stele. 
It has already been pointed out (Browne, p. 679) that even in the species 
in which the sporangiophores alternated more or less regularly externally, 
there was no such regular alternation of the traces at their points of origin 
on the axial stele. When a parenchymatous mesh extends into more than 
two internodes the neighbouring traces cannot be accurately superposed 
to those of the second whorl in a downward direction, as they would be if 
the alternation of successive whorls were regular. In a cone, therefore, like 
that of E. maximum , in which a considerable number of meshes extend into 
more than two internodes, cases of regular anatomical alternation are not 
very common, even over a limited area. The more reduced the xylem-system 
the rarer such cases will be. Thus we find in Cone A a certain number of 
traces alternating rather regularly with the corresponding traces of the 
whorl below, and sometimes also with those of the whorl above. In 
Cone B such cases are relatively rarer. Cases of what I have elsewhere 
termed irregular alternation (Browne, pp. 681-2) are common both in 
Cone A and in Cone B. There are four forms of irregular alternation : 
(1) The xylem strand of an internode may be formed by the fusion of 
a branch of a forking strand with a whole strand of the internode below; 
or (2) by the fusion of two whole strands ; or (3) by the forking of a single 
strand above the departure of a median trace, one or (4) both of the resulting 
strands giving off a trace. Of these modes of irregular alternation the first 
is by far the commonest, while the last is very rare. 
More or less regular superposition of traces of successive whorls occurs 
commonly in cones of E. maximum , as it does in those of E. limosum , and 
more rarely in those of E. palustre and E. arvense . It arises in the same 
way as in those species. The parenchymatous meshes on either side 
of a strand extend upwards through several internodes, and as the strand 
thus pursues an isolated course through several nodes, the traces given off 
from it are necessarily superposed. This superposition is clearly due 
to reduction of the vascular system. When the meshes persisting on either 
side of the trace-bearing strand are narrowed by the formation of a certain 
amount of additional xylem at the node, the traces of successive whorls are 
not necessarily accurately superposed, since it is possible for successive 
traces to depart from different sides of the strand ; but where the reduction 
of xylem at the node is greater, and the strand remains relatively narrow, 
the superposition of traces given off by it at successive whorls is necessarily 
more accurate. In Cone A, in which the xylem is relatively well developed, 
a single strand does not seem to give off more than three, or at most four, 
consecutive and superposed traces ; but in Cone B, an isolated strand some- 
