62 
N. Bancroft. 
direction by adopting a collateral structure” in their extrafascicular 
rings. 
In the primary node of Macrozamia Denisonii (22), there are 
cortical cauline strands wholly or partly concentric. Worsdell 
believes these to be homologous with the anomalous concentric 
strands in Medullosa anglica and with the extrafascicular strands 
figured by Weber and Sterzel (17, see Taf. iv, Fig. 2, b) in Medullosa 
Leuckarti (Fig. 16). 
In Medullosa Solmsii and Medullosa Leuckarti, it will be 
remembered, there are two primary rings of steles, those of the 
innermost ring being smaller and less well-organised than those of 
the outer. Medullosa porosa also possesses an inner ring, the steles 
of which may have been derived from the medullary system. “ In 
fact, transitions exist between the medullary system and the inner 
Fig. 16.—A , Macrozamia Denisonii. Transverse section of concentric cortical 
strand, just outside the central cylinder of primary node. cf. Medullosa Leuckarti. 
B, Medullosa Leuckarti. Transverse section of concentric strand outside 
the central cylinder of the stem. (Adapted from Worsdell, 1901). 
cylindrical system of steles.” Worsdell is convinced “that in the 
Medulloseae, the cylindrical and medullary systems are merely 
variants of a single system ” ; either the cylindrical system has been 
formed by the regular annular grouping of the outermost members 
of a system of scattered steles, in which case the medullary strands 
would represent the primitive arrangement: or, the medullary steles 
have been derived from a primitive cylindrical system. According 
to Worsdell, the latter alternative is the most tenable. 
Usually the medullary steles are concentric in structure, but 
those in Medullosa porosa show various stages of reduction towards 
the collateral type, orientation being irregular. 
The medullary system of Encephalartos and some Macrozamias 
is comparable with the medullary system in Medtillosa, being of the 
same system as the vascular rings. 
