ee  Medullosez. By tracing these strands upwards or downwards, they are seen to be in ` 
118 MR. W. C. WORSDELL ON THE 
of the stem, internal to the large, elongated steles forming a kind of central cylinder, 
are scattered innumerable tiny steles united together into an anastomosing network; 
these minute steles have, almost certainly, arisen in the past by the breaking up of a 
few large steles. In modern Cycads the anastomosing system of medullary bundles 
characteristic of Encephalartos and certain species of Macrozamia has had, in all 
probability, in the far past, a similar origin to the internal central bundles of Medullosa 
and the fern Dicksonia. On this view of the direct homology existing between the 
central cylinder and the medullary strands, we can understand why the latter should be 
present in some species of Macrozamia aud absent in others, for this would be merely 
equivalent to the presence of, say, two extrafascicular vascular zones in one species, and 
the presence of only one such zone in another species; the presence or absence in these 
plants of medullary strands is, in short, a character dependent on a variable amount of 
development of the normal vascular strands. If the medullary bundles, like the 
leaf-traces, constituted a system of their own distinet from that of the vascular 
cylinders, it would be strange and inexplicable that this system should be present in one 
species of Macrozamia and absent in another, as I have found to be the case. 
In the light of the above view, however, the origin and homologies of the medullary- 
bundle-system in modern Cycads and the Medullosez are made plain: they are the 
broken-up fragments of a ring or irregular group of steles forming the innermost or 
central portion of the vascular system of the stem. 
My fig. 10 (Pl. 16) of a portion of the cylinder of the sporophyll-bearing region of the 
cone-axis of a male plant of Ceratozamia mexicana, Brongn., and fig. 11 of the entire 
cylinder in C. latifolia, Miq., represent the structure pertaining to the first of the 
above-named types, there being no other bundles in the pith except those constituting 
the intrafascieular ring. 
The bundles composing the intrafascicular rings in fig. 1 & 2 of pl. 7 aud fig. 2 of pl. 2 
of Weber & Sterzel’s paper are seen to be more or less incomplete in structure, a 
portion of the otherwise concentric bundle being lost either from the inner (as in the 
first two figures above cited) or from the outer side (as in the last-cited figure). 
Now my fig. 11 shows strands belonging to this intrafascicular ring in which the 
reduction has gone still further, so that as a rule, only a very small segment of what 
was once (in the plant's ancestors) a perfect concentric strand (such as is seen in 
pl. 2. fig. 1 of the above-mentioned paper) has been left remaining, and, from the present 
orientation of this bundle, it can be clearly determined to which region of the original 
concentric strand it belonged, The least reduced condition of the latter is represented ` 
by a case in my fig. 11, where three bundles (two smaller and one large one) are closely 
approximated and with their xylems mutually directed towards each other. In fig. 10 
it is (in three out of four cases) the outermost portion of the former concentric strand 
which is left remaining, one of which portions almost equals in size half of that of the 
original strand. But I have also found cases where the complete concentric structure ` 
has been preserved (fig. 12), such a strand being the exact counterpart and homologue 
of the complete concentric strands occurring in the same region of the stem in the 
