148 W or s dell . — T he Structure and Origin of the Cycadaceae. 
beginnings of the second, third, and fourth c extrafascicular ’ cylinders 
respectively ; it will be noted that two of them exhibit traces of the 
primitive concentric or stelic structure. 
In the basal region of a mature stem of Macrozamia Fraseri I observed 
small, rudimentary inverted strands on the inner side of the first and second 
‘extrafascicular’ rings; these, doubtless, represent the corresponding 
portion of the concentric strands which were seen to occur immediately 
outside the central cylinder in the same region of the stem in a young 
plant of M. Denisonii ; they are very interesting as showing how such 
ancestral and (for modern requirements) unnecessary parts of the structure 
become swamped and rendered utterly obscure as individual growth proceeds. 
In the same region of the stem of Cycas Seemanni I found the first 
‘ extrafascicular ’ ring to be composed of concentric strands situated at wide 
tangential distances apart, which differed from those composing the corre- 
sponding ring in Macrozamia and Encephalartos in having a symmetrical 
contour, the tissues being of the same thickness all round ; the ring next 
outside this formed a continuous zone having a collateral structure. In the 
same region of the axis of another individual but at a slightly lower level, 
viz. where root-structure of the central cylinder occurred, the first ‘ extra- 
fascicular ’ ring consisted of a much broken-up collateral zone ; one or two 
of the isolated strands possessed an almost equal amount of inverted xylem 
and phloem on the inner side. The second extrafascicular ring is a con- 
tinuous thick collateral zone with, at intervals, some extremely minute , 
rudimentary , inverted bundles on its inner side; these represent the last 
faint traces of the former solenostelic structure of this ring. The third ring 
exhibited much the same characters as the first. 
In all species of Cycas the stem possesses throughout its length 
a cortical system of perfectly concentric secondary strands which are cauline , 
terminating above in the base of a leaf and connected, during their course, 
with incoming leaf-traces. They appear to be peculiar to the genus Cycas , 
and to have no exact counterpart in the Pteridosperms. In any case, they 
are interesting ; for they constitute an outlier from the main conducting- 
system of the stem, and hence have escaped all those modern influences 
which have reduced the ancestral concentric structure of the main rings for 
the most part to the collateral type. This outlying ring of concentric 
strands resembles precisely the first * extrafascicular ’ ring of strands 
observed by me in the case of the stem of C. Seemanni , and also the 
‘ accessory vascular strands ’ described by Scott in the stem of Med. anglica. 
In all these cases the central, irregularly-scattered tracheides, which so 
greatly resemble primary elements, are in reality the first-formed secondary 
tracheides of the wood of the stele. The same isodiametric elements can be 
found on the inner side of the ‘extrafascicular’ collateral rings in all Cycads 
which possess them. 
