THE MORPHOLOGY OP THE STELE OF PLATYZOMA MICROPHYLLUM, R. BR. 587 
the outer stelar xylem, would occupy the positions indicated by the three crosses 
in text-fig. 3, and if projected into the stele itself, could not fail to have a 
mesarch position. The theoretical position of the stem-protoxylems would therefore 
be mesarch. 
It will be evident that the tubular portions of the stele of Platyzoma are con- 
structed on the plan indicated in text-fig. 3, with outer and inner endodermis 
(O.E. and I.E.), between which lie in centripetal sequence pericycle (P.), phloem (Ph.), 
outer xylem (O.X.), inner xylem (I.X.), and inner parenchyma. Internal to the 
inner endodermis is what is generally recognised as the pith (M.), but it will be clear 
ax, 
Text-fig. 3. — Diagram of structure of medullated stele of Platyzoma as seen 
in transverse section. O.E., outer endodermis ; I.E., inner endodermis ; 
P., pericycle; Ph., phloem; O.X., outer xylem; I.X., inner xylem; 
■ M., medulla. ( x 50. ) The three crosses indicate the positions of the 
leaf- trace protoxylems. 
that if the 'pith as a whole is in this case of purely intrastelar origin , the inner 
parenchyma immediately within the parenchymatous inner xylem is a portion of the 
original pith which has not been included within the inner endodermis. On the 
other hand, if the pith were of cortical origin, then only the ground-tissue enclosed 
by the inner endodermis could reasonably be regarded as pith, and the inner paren- 
chyma above referred to would be open to interpretation as the intrusive extension of 
the pericycle. There seems no ground for the latter belief in the facts now recorded. 
The outer and inner endodermal cylinders have been shown to be entirely inde- 
pendent of each other, the former being invariably maintained inviolate while the 
latter has fluctuated and locally vanished, and neither fully developed nor degenerate 
stelar gaps have been found. Throughout the entire plant which has just been con- 
sidered the tissues were apparently normal in form and differentiation, and no grounds 
were found for considering the plant abnormal or perverted in any of its parts. 
TRANS. ROY. SOC. EDIN., VOL. LIT, PART III (NO. 22). 89 
