58 N. Bancroft. 
are to be regarded as monostelic; he explains the additional 
vascular cylinders of some recent Cycads as being “ local 
peculiarities in the vascular system due to anomalous distribution 
of the cambium”; they are therefore of no phylogenetic significance. 
Fig. 12 ,—Medullosa Leuckavti. Transverse section of part of stem, pv', 
first peripheral vascular system ; pv 2 , second peripheral system ; sv, medullary 
system of star-rings. (Adapted from Weber and Sterzel, 1896). 
According to Worsdell, the normal vascular system of Cycads 
consists of “the one-sided remnants of a number of steles”; and 
the so-called anomalous vascular structures in some genera are all 
that remain of concentric cylinders, normal in an ancestral type. 
Worsdell bases his theory of the Medullosean ancestry of Cycads 
