144 WorsdelL — The Structure and Origin of the Cycadaceae . 
the medullary system and the inner cylindrical system of steles. I hold, 
further, that in the Medulloseae the cylindrical and medullary systems are 
merely variants of a single system. That the annular or cylindrical system 
has either been derived in the past from a primitive system of irregularly 
scattered steles, of which the medullary strands are the surviving remnant ; 
or, on the other hand, that the cylindrical system is primitive, and has 
itself given origin to the medullary system. I am inclined to regard the 
latter as the most probable view. 
In most cases the medullary steles are perfectly concentric in structure, 
but this is not always so ; e.g. in M. porosa the outermost, as also several 
of the more central, steles are imperfectly concentric ; in such cases the 
resultant collateral or sub-concentric stele is very variously orientated, 
according as the secondary tissue is defective on this or that side of it. 
Now in the modern 
genera Encephalartos and 
some species of Macro- 
zamia there is a medullary 
system of, usually, very 
small and very numerous 
collateral bundles of ex- 
tremely various and ir- 
regular orientation ; this 
latter may be primarily 
due to the causes just 
mentioned in the case of 
Medullosa ; but appears 
more directly traceable to 
the fact that each bundle 
follows the sinuous course 
of a mucilage-duct to- 
wards which its phloem is always orientated. But the cauline origin of 
these medullary bundles may be attributed to the fact that they constitute 
one and the same system with those of the vascular rings. 
In the fertile part of the axis of the male cones of Ceratozamia 
mexicana , Brongn., and C. latifolia , Miq., I observed the rudiment of an 
in trafascicular primary cylinder of bundles ; these latter were much less 
developed than those of the outer cylinder ; they were in one of the species 
arranged frequently in small groups of three (Fig. io), representing small 
fragmented concentric structures, for their xylems were mutually orientated 
towards each other ; other isolated bundles were inversely orientated ; in the 
other species the inner bundles were much less numerous and usually 
rudimentary in development, staining badly ; one such was observed to be 
perfectly concentric in structure and to end blindly upwards. In both 
