430 Br ozone. — Anatomy of the Cone and Stem of Equisetum. 
the latter outwards or laterally being hindered by the large size already 
attained by the cells of the sheath, the direction of bonification of any 
further tracheides might be bound to follow more or less the outline of the 
sheath inwards. 1 The disposition of these more deeply seated, large 
tracheides and their appearance, as we pass upwards, in continuity with the 
ordinary centrifugal metaxylem strongly suggest such an explanation. 2 
But though this is the general impression left by their appearance and 
position, it is of course possible that these elements are the remains of 
ancestral centripetal xylem. 
Pfitzer (pp. 310-11, 313) states that there are common inner and outer 
endodermes both in the stem and the rhizome of E. variegatum . In the 
cone, however, the separate bundles are surrounded by special, very distinct 
endodermes. When two bundles fuse, or when a strand increases in width, 
when it narrows or branches, the endodermis takes the outline of the vascular 
tissue (cf. PL XXI, Figs. 4, 5, and 8). Consequently the distribution 
of the endodermal cells is always varying, and they cannot be held to 
possess any morphological or phylogenetic importance. 
III. The Course of the Strands in the Cone. 
The network of strands in the cone of E. sylvaticum recalls that of the 
cones of E. maximum , especially of the smaller specimens with their 
relatively better developed vascular system. The reconstructions of the 
steles of Cones A and B of E. sylvaticum (Text-figs. 1 and 2) are very similar 
to the reconstruction of the stele of Cone A of E. maximum (cf. Browne ( 2 ), 
PI. XII), allowance being made for the different scales of the figures and for 
the larger size and more numerous component parts of the cone in E. maxi- 
mum . In Cone A of E. sylvaticum it is rather difficult to estimate the 
number of parenchymatous meshes that arise within the cone (i. e. above the 
sporangiophores), because in it the annulus is attached very close to the points 
of insertion of the lowest sporangiophores. The latter are disposed a little 
irregularly and at slightly different levels. As a result of the absence 
of elongation in the region between the annulus and the lowest fertile whorl 
the parenchymatous meshes arising by the supra-annular branching of the 
axial strands originate but very little below those arising above — in most 
cases but little above — the points of departure of the traces of the lowest 
sporangiophores. Thus, on a superficial examination of Text-fig. 1, two of 
the meshes — those shown in the reconstruction as arising between and slightly 
below the first and fifteenth, and the second and third traces respectively of 
1 Such an explanation would not involve the attribution of any morphological value to the- 
sheath, since the suggested modification in the direction of development is due to a purely 
physiological character, the early increase in size of the cells of the sheath. 
2 A somewhat analogous origin of ontogenetically centripetal from phylogenetically centrifugal 
xylem has been suggested by Chodat (1908) for the mesarch strands of Lyginodendron. 
