6i6 W or s del L — Anatomy of Stem of Macrozamia 
between them being the dead, brown, parenchymatous tissue 
of the leaf-base, in the still living portion of which latter the 
leaf- traces are seen passing inwards to the cortex. After 
the leaf-bases have been disposed of, the periderm continues 
to eat its way into the cortex. Its successive layers arise 
in a very irregular manner, and often in the phelloderm of 
the older one (Fig. 17). There is no clean excision of the 
tissues along a definite line, as in Zamia , whereby a smooth 
even bark is obtained ; but the outer surface of the stem, 
owing to this irregular sinuous formation of the periderm, is 
left very rough and jagged, consisting either of the remains 
of the leaf-bases or of dead portions of the cortex. 
Owing to the periderm-formation it is difficult, in an old 
stem, always to obtain a proper clue as to the course of many 
of the leaf-traces in the extreme outer part of the cortex, 
as this region is often entirely cut away. 
Summary. 
The most noteworthy characters exhibited by the anatomical 
structure of the stem of Macrozamia are the following : — 
1. The well-developed medullary system of bundles , which 
has a distinct adventitious origin of its own and is not directly 
continuous with the primary leaf-trace system, but only forms 
secondary anastomoses with it, being connected, by means 
of the medullary rays, with the various parts of the vascular 
ring. The curious contortions and involutions which some 
of the bundles of the pith undergo are also to be noted. 
2. The anomalous zones of secondary thickening which, to 
the number of two or three, surround the normal cylinder. 
Of these the innermost is the best developed, being as thick 
as the normal ring upon which it immediately borders, and is 
the only one quite readily distinguishable with the naked eye 
on the cut surface of a stem. The second and third rings 
are much more feebly developed, and, in the upper part of 
the stem, often entirely absent. They are usually composed 
