COMPARATIVE ANATOMY 339 
anomalies, finds its nearest parallel in certain fossils belonging to the 
Lycopodiales. 
It may thus be concluded from comparative examination of all the 
leading types of the Lycopodiales that the vascular structure of the mature 
shoot is referable in origin in all cases to the non-medullated monostele. 
This is actually seen existent in the stems of Se/agznella spinulosa, though 
in its lower portion the protoxylem is central; but it is also shown more 
amply developed, and with the protoxylem in the accustomed position at 
the periphery in the upper region of that plant, as well as in certain 
stems of Lepzdodendron. Comparative study of the Lycopodiales shows 
that all the variants of vascular structure known in them may be referred 
in origin to this simple type. In Lycopodium the modification has been 
by intrusion of the phloem more or less deeply into the xylem-core, till 
this may at last be divided into distinct plates, or riddled like a sponge. 
In Selaginella there is amplification in various ways, the most obvious 
being by the adoption ofa solenostelic structure, or more commonly by 
segregation of the enlarging stele to form a varying number of meri 
steles. Among the dendroid fossils, where the demands on the conducting 
system were Jarge in consequence of the large size of the plants, the 
extended stele became first medullated, as seen in most stems of Lepido- 
dendron: and then in the later Szgz//arzas the residual ring of xylem 
became broken up into more or less distinct strands. In these types 
additional vascular tissue was supplied by the potentially unlimited 
developments from an external cambium. Finally, in Isoetes a complicated 
structure, partly primary, partly secondary, is found, which would be 
hardly intelligible except when studied in the light of the dendroid fossils ; 
but even this, in common with the rest, is referable in origin to the 
non-medullated monostelic type, together with the results of secondary 
thickening. The bearing which this constant reference to a primitive 
monostele has upon a strobiloid theory is plain: as is also the fact that 
throughout the Lycopodiales the foliar traces are inserted peripherally, and 
with only slight local disturbance upon the periphery of the cauline 
xylem-core: for this indicates structurally that the leaf is in them all the 
minor, while the axis is the dominant feature of the shoot. 
Scott and Hill rightly point out that the view of the central cylinder 
as cauline applies only to the adult stem of /soefes: in embryonic stages 
the construction of the vascular system is from the union of definite 
leaf-traces: this is the case also in the embryonic stages of certain other 
Lycopods. The question of the relation of these facts to a theory of 
the strobilus will be taken up in connection with the embryology of the 
Lycopods, which forms the subject of the next chapter. 
