138 Worsdell, — T he Structure and Origin of the Cycadaceae. 
fragmentation of a few (two or three) steles of extremely sinuous contour, 
such as are found in the stem of Medidlosa Leuckarti , Gopp. and Stenz. 
(Fig. 3). If all the bundles, instead of only one or two, resulting from this 
fragmentation had remained concentric, we should have a structure like 
Fig. 3. Medullosa Leuckarti: transverse section of vascular system of stem, schematically 
drawn to represent how the vascular structure of Stangeria (illustrated above) could have been 
derived by fragmentation of the large irregularly shaped steles, the dotted outlines representing 
the eliminated portions. 
■F 
that of M, Solmsii (Fig. 4), whose polystelic character I regard as probably 
derived from the solenostelic condition of such a form as M. porosa ; on 
this view the structure of M. Leuckarti would represent a kind of 
intermediate stage between the two. 
The incurved and horse-shoe-shaped bundles of Stangeria , I am 
convinced, possess a great phylogenetic and morphological significance : 
they are the remains of the concentric bundles or steles of the Medullosean 
ancestor. This easy explanation of 
the phenomenon seems to me the only 
one possible. It is only in this lower 
region of the peduncle that this an- 
cestral trait can maintain itself ; higher 
up the bundles become orientated in 
the normal manner in order to adapt 
secJnVp themselves to the modern economic 
inner primary ring ; pr 2 — . outer necessities of conduction. Higher up 
also, the centripetal xylem appears more 
in evidence ; this too, with Scott, I 
regard as an ancestral character, as its tardy appearance in the ontogeny 
would help to indicate. In the concentric and horse-shoe-shaped bundles 
of the lower region it is absent, the available space for such a tissue 
stem ; pr 1 
primary ring (reversed and in part restored) 
(after Weber and Sterzel). 
