Relation of Pteridosperm Anatomy to that of Cycads. 55 
three zones of normally orientated xylem and phloem outside the 
ordinary peripheral system, just as in some recent Cycads there is 
a development of extrafascicular zones of vascular tissues. 
Medullosa porosa (Fig. 10) and its varieties are very similar to 
M. stellata. In this case, the peripheral vascular system forms an 
unbroken, or only incompletely broken ring, like a solenostele. The 
outer medullary- or star-rings are often incompletely surrounded by 
secondary wood, this indicating a tendency towards reduction of the 
Fig. 10. —Medullosa porosa v. typica. Transverse section of stem, showing 
pr, the unbroken peripheral vascular system ; sr', the outer series of star-rings 
with incomplete development of secondary tissues ; sr", the inner star-rings. 
(Adapted from Weber and Sterzel, 1896). 
concentric steles to form collateral bundles—an important tendency, 
when a comparison is made with recent Cycads, according to 
Worsdell. 
In the form-cycle of Medullosa Solutsii (Fig. 11), the peripheral 
system consists of two zones of plate-rings. There are usually 
numerous very small star-rings. The variations depend chiefly 
upon the differences in development of the normal and inverse 
xylem of the outer plate-rings. In variety liguosa (Fig. 11, c ), there 
are several consecutive zones of normally orientated wood and 
phloem as in M. stellata v. gigantea and some recent Cycads, 
