146 Worsdell. — The Structure and Origin of the Cycadaceae. 
approximation to a concentric structure V Here we see clearly preserved 
in the cone-axis traces of a second cylinder. This latter has long since 
disappeared from the vegetative axis, Ceratozamia being one of the mono- 
cylindric forms. 
Let me now pass to the consideration of the ‘ extrafascicular ’ secondary 
strands and zones ; they occur, as mentioned above, in the stem of four 
genera of modern Cycads, and in that of Medullosa stellata , var. gigantea. 
The structure of this latter is almost exactly a replica of that of a modern 
Cycad which possesses it. I will therefore consider them together. At 
the outset I may say that I regard all the cylinders or rings as mutually 
homologous , and that there exists no morphological distinction between the 
primary and secondary zones. The comparatively late period at which all 
vascular rings except the first arise necessitates their earliest beginnings 
being inaugurated by a cambium\ but this does not render them in any 
way different in nature from the first-formed one. What is, therefore, the 
origin (which must be the same for all) of these zones ? 
In certain Medulloseae, as in M. stellata , var. lignosa , M. Solmsii, vars. 
incrassata and lignosa , the outer part of the secondary wood of the primary 
cylinder has far exceeded in thickness that of the innermost portion which 
has become, as it were, left behind on the road of advancement. In 
explanation of this phenomenon I may quote my own words of nine years 
ago 1 2 : ‘ 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 primitive condition, in which the secondary wood was of equal 
development all round or on both sides, is seen in the polystelic cylinder 
of M. Solmsii (Fig. 4), and in the solenostelic cylinder of M. porosa 
respectively. 
In the case of the Medulloseae the innermost part of the secondary 
wood of the primary ring, never, as far as we know, became extinct ; in 
modern Cycads, on the contrary, it became, along with the phloem (except 
in the case of Encephalartos Barteri , described above), completely extinct. 
As regards the outer secondary rings , it is a remarkable fact that these, 
in the stem of Med. stellata, var. gigantea, had, apparently, already com- 
1 Matte describes the same phenomenon in the peduncle of Bowenia and Ceratozamia. In the 
latter he also figures concentric and arc-shaped bundles in this position. 
2 Dec. 1896. 
