Vascular System of the Genus Equisetum . 223 
development is quite independent of those in the internodes above and 
below, with which they are subsequently connected by the nodal tracheides. 
In the cone, however, the protoxylem strands develop uninterruptedly from 
the base to the apex of the cone. There is no disjunction at the * nodes 
though some of the strands may terminate at or near the point of insertion 
of the sporangiophores. Moreover, the traces of the latter arise in 
a manner quite different from those of the leaves. The differentiation of 
the tracheides of the leaf-traces begins in the distal part of the leaf and 
proceeds inwards until they finally link up with the nodal tracheides. In 
the sporangiophore trace the order of development is reversed. The first 
tracheide differentiates next to an axial strand of protoxylem, to which it 
may be connected laterally or terminally, and the further differentiation of 
the strand proceeds outwards into the stalk of the sporangiophore (Text- 
19)- 
In any comparison of the vascular structure of the cones and vegeta- 
tive shoots, such marked difference in the arrangement and mode of 
development of the protoxylem strands must receive full consideration. 
They are features which would be least affected and longest retained in any 
process of reduction. In the vegetative shoots the leaf-sheaths, though now 
of comparatively small importance in the economy of the plant, have 
a dominating influence on the. vascular structure of the stem. The sporan- 
giophores, though provided with a vascular system of relatively better 
development than that of the leaf-teeth, have no similar effect on the 
anatomy of the cone. 
The examination of mature cones of such species as E. arvense and 
E . maximum , with more or less regular whorls of sporangiophores and with 
well-developed metaxylem masking the protoxylem strands, may perhaps 
suggest an internal organization of node and internode. A study of the 
development of the vascular system, however, can only lead to the conclu- 
sion that the vascular structure so characteristic of all the mature vegetative 
axes, with its well-defined segmentation, is quite absent in the cone. 
With the appearance of metaxylem tracheides in the axis — which 
takes place somewhat irregularly — additional elements are added to the 
sporangiophore traces, which when completed show a mesarch structure as 
described by Eames ( 19 ). The axial metaxylem consists of spirally 
thickened tracheides which vary considerably in length. The advantage of 
this type of tracheide is evident in view of the rapid elongation of the cone 
which takes place at the time of dehiscence of the sporangia. 
As seen in transverse sections (Text-figs. 20 and 21), the metaxylem is 
separated from the protoxylem by parenchymatous cells, and as the cone 
matures it forms a continuous band, 2 to 4 cells wide, external to the 
protoxylem in each bundle. The greater development of the metaxylem 
tends to mask the presence of the protoxylem. The latter, however, is 
Q 
