230 Barratt . — A Contribution to our Knowledge of the 
parenchymatous cells. The protoxylem is poorly developed and the 
elements not ruptured, hence a carinal canal is absent. Queva’s figure 
( 7 , p. 135) of a bundle from E. litorale may be compared with Text-fig. 9. 
We obtain, however, still further enlightenment from an examination of 
the cone. 
In the latter organ the predominance of metaxylem is very striking, 
the protoxylem being represented by relatively few and small 'elements well 
to the interior of the irregular bundles. The order of differentiation in the 
following species, E. arvense , E. maximum , E. palustre , E. limosum , 
E. sylvaticum , was undoubtedly centrifugal, but the existence of mesarch 
traces in the sporangiophores gives credence to Eames’s (10) view that the 
cones o i E. hie male and E.Jluviatile 'exhibit mesarch axial strands. 
Thus the internodal bundles of the aerial vegetative axis are seen to 
conform in structure with those of the tuber and cone. The lateral strands 
A B - C * 
Text-fig. 24. Diagram showing bundles from: A, cone; B, abnormal tuber; C, normal 
tuber ; D, internode of stem. 
are the relics of a continuous band of metaxylem such as now exists in the 
bundles of the cone and less perfectly in those of the tuber (Text-fig. 24). 
A somewhat similar suggestion was put forward by Browne ( 1 ), who, 
however, regarded the carinal tracheides as representing the middle portion 
of the once continuous xylem. It has been shown above that in all cases 
the protoxylem forms an independent strand always internal to the meta- 
xylem. 
As Browne suggests, the vascular structure of the cones of some 
species shows a reduction series, much more xylem being present in the 
cones of E. arvense, for example, than in those of E. limosum. It is quite 
clear from an examination of the former of these species that the basal 
plan was a siphonostele in which gaps have arisen owing to the failure of 
the cells to differentiate as tracheides. On this conception the structure 
met with in the cone links together the unbroken siphonostele of the basal 
region of the young plant and the vascular structures of the node and inter- 
node of the vegetative axis. 
