249 
Cone and Fertile Stem of Eqnisetum . 
the internodal stele assumes the appearance of a ring broken only in two 
places by a single, small, separate strand (PL VIII, Fig. 6). In E. hyemale 
it is rare for the strands to become united much below the level of the 
fertile node. 
Taking into consideration the various factors that contribute to the 
development of the vascular system in the cones of Eqnisetum it is clear 
that the species in which the xylem of the cone is relatively best developed 
is E. arvense . The cones in which the axial xylem is relatively most 
reduced are those of E* maximum and E. limosum — the xylem of the cone 
of the latter being, it would seem, comparatively slightly more reduced than 
that of the cone of the former. The cones of E. hyemale , E. palustre, and 
E. giganteum represent the middle term of such a series. The cones men- 
tioned do not, of course, constitute a phylogenetic series. On the whole the 
vascular system of the cone of E. pain sire is rather less well developed than 
that of E. hyemale. A comparative estimate of the reduction of the stele of 
the cone in E. giganteum on the one hand, and in E. hyemale and E.palustre 
on the other — or indeed of the vascular system of the cone of E . giganteum 
and that of other species of the genus — is very difficult to carry through. 
This is because in the cones of E. giganteum two opposite tendencies are at 
work : firstly, a tendency for the amount of xylem developed to be compara- 
tively large, owing chiefly to the formation of wide internodal sweeps of 
xylem, but also partly to the closure of parenchymatous meshes some 
distance below the node and to the radial extent of the xylem ; secondly, 
a tendency for relatively little xylem to be developed owing to the per- 
sistence of some parenchymatous meshes through a considerable number of 
internodes. Neither tendency is peculiar to E. giganteum , though in no 
other among the species studied is the formation of internodal sweeps of 
xylem so marked a character. It is the combination of these two tendencies 
that makes it difficult to estimate the position of this species in a series 
showing gradual reduction of the xylem. Perhaps its cone may be best 
regarded as having relatively more xylem than that of E. palustre , but 
comparatively a little less than that of E. hyemale . 
IV. Alternation and Superposition of the Traces. 
At the periphery of the axis of the cones of E. hyemale and E. gigan- 
teum , the sporangiophores of successive whorls alternate regularly with one 
another, except where there is a change in the number of members. Even 
in the latter case the area of disturbance of alternation is generally restricted. 
Usually the young sporangiophores are closely imbricated, their peltate 
expansions being accurately fitted in with those of the whorls above and 
below. No such regular alternation of the traces prevails at their insertion 
on the stele. The persistence of parenchymatous meshes through more 
than two internodes, of course, makes it impossible for the traces of a whorl 
