229 
Vascular System of the Genus Equisetum. 
condition does not persist for long ; it is temporarily disturbed by the 
attachment of the first lateral branch, and above the first node the solid 
cylinder opens out at once into the much-reduced structure of the internode. 
Such a protostelic condition is not met with in the succeeding shoots. A 
very characteristic and significant vascular formation is found in the com- 
pound structure formed from the bases of the first few aerial shoots. This 
is a continuous sympodial siphonostele uninterrupted by leaf-traces or 
parenchymatous gaps of any kind. It is also significant that a siphonostele 
is to be found at the base of every branch, whether borne above or below 
ground, and that it is repeated at every node of the vegetative shoots. 
The internode undoubtedly shows a much reduced vascular develop- 
ment, and we must look for an explanation of its characteristic structure in 
the plant organs which have suffered less reduction. The structure of the 
basal regions already referred to above provides a very strong argument for 
a siphonostelic origin, and there seems to be small reason to doubt that the 
bundle arrangement now present in the internodes is the final stage in 
reduction from such a condition. 
Gwynne-Vaughan (9) first raised the question as to the true nature of 
the lateral strands of internodal xylem, and Poirault (22) agreed with him 
in his conclusion that they had no connexion with the leaf-trace system, but 
were continuous over the nodes, and that their development was in a centri- 
petal direction. 
Gwynne-Vaughan suggested that these strands may represent the last 
remnants of a central mass of centripetal xylem. This view was seriously 
attacked by Eames (10), who not only thought that the whole xylem of 
each bundle made up a unit, but that all parts were involved in the forma- 
tion of the leaf-traces. 
From the present investigation it seems certain that the order of 
differentiation of the elements of the lateral strands is in general centri- 
fugal, as the majority of the later investigators (Qudva (7), Browne (1)) have 
agreed. Eames (10), however, seems to have overlooked the fact which 
Janczewski (8) and Queva (7) had previously noted, that the xylem of each 
internode develops quite independently, only linking up subsequently by 
the development of the nodal tracheides. He, in fact, speaks of the carinal 
canals as disappearing at the nodes and their place being occupied by large- 
celled protoxylem. The protoxylem certainly does not traverse the node, 
and no elements of that region can be described in such terms, since they 
are all alike in character, agreeing in their short length and in bearing 
reticulate thickenings. 
The true nature and relations of these various strands of xylem can be 
seen by reference to other organs. In the first place the bundles of the 
tubers give us a clue. There the xylem is not separated into three strands, 
but forms a continuous group of tracheides, though often interspersed with 
