6 1 8 Worsdell. — Anatomy of Stem of Macro zamia 
Medullosae 1 , a fossil group with many Cycadean affinities ; 
but it would be rash and premature to suggest here a homo- 
logy between the two. 
From these considerations I have been led to give the 
following provisional suggestion, viz. — 
That the anomalous structures in Cycadean stems, and 
especially those in the stem of Macro zamia, are remnants 
of some ancient structure once common to a large group 
of plants. That this structure consisted of rings or layers of 
concentric vascular strands. That, as time went on, and 
greater specialization in the conducting-tissues arose, and 
a need for the formation of a larger amount of this tissue 
became urgent, the cambium of the inner portion of each such 
concentric strand gradually became less and less functional, that 
of the outer portion, on the contrary, more and more active, 
so that a much larger quantity of wood and bast became 
formed on the outer side of each strand than on the inner 
side, for this was the surest and best means of economizing 
both space and expenditure in the building up of an efficient 
conducting tissue for the stem. 
The result is, finally, the structure, as we at present know 
it, in the stem of Cycas, Encephalartos , and Macrozamia. 
Some evidence for its origin lies in the relics of the inner 
cambium of the concentric strands in the vascular zones of 
Macrozamia ; in the ‘ transfusion-tracheides ’ constantly found 
on the inner margin of every ring both of Cycas and Macro- 
zamia ; and in the presence of rings of concentric bundles, 
still preserving the old structure, in the cortex of Cycas. 
5. The presence of leaf-traces in the cortex which run 
directly inward to the vascular rings after a very short 
tangential course, without describing the curves characteristic 
of the girdles in this and other Cycads. 
6. The structure of the girdles during their tangential 
1 Goppertu. Stenzel, ‘ Die Medulloseae, eine neue Gruppe der fossilen Cycadeen,’ 
Palaeontographia, Vol. 28, 1881. Sterzel u. Weber, ‘Beitrage zur Kenntniss der 
Medulloseae,’ XIII. Berichtder Naturwissenschaftlichen Gesellschaft zu Chemnitz, 
1893-1896. 
