586 
DR JOHN M‘LEAN THOMPSON ON 
indications of stelar gaps of any kind were found. The same remark applies to the 
apical portion of the stem which remains to be considered. 
This younger portion of the stem retained to its apex the well-defined tubular 
stele, and accordingly only -a small part of its stele is represented in text-fig. 2. 
With a number of other apical portions of stems it was examined throughout so as 
to demonstrate the steps of stelar differentiation from the apical meristem. In all 
essential points the differentiation which it showed was in agreement with what 
was seen in the other materials. The description given for it will suffice for all. 
The stelar structure may conveniently be read backwards from the apex to the point 
of complete stelar differentiation. In this way the entire stelar structure of the 
plant under consideration will have been described. 
Close to the apex of the stem the stele appeared in section as in fig. 25. “Both 
Text-fig. 2. — Diagram of the structure of a portion of the stele of the small plant of Platyzoma described in the text and 
illustrated in the plates. Both outer and inner endodermis are represented by black lines ; phloem is hatched and xylem is 
solid black. Pericycle, inner parenchyma, and pith are white. The diagram represents the general arrangement of tissues 
as seen in what would have been a median longitudinal section of the stele. ( x 16.) 
outer and inner endodermal sheaths were clearly defined. Between them lay the 
still imperfectly differentiated pericycle, phloem, xylem, and inner parenchyma. 
Xylem-differentiation had commenced intermittently at the periphery of the xylem 
and in the nascent traces, but the inner xylem was not yet clearly distinguishable. 
No stelar protoxylems were seen. Further back the condition represented in fig. 24 
was found. The outer xylem was almost completely differentiated, but though the 
large tracheides of the parenchymatous inner xylem were clearly indicated, their 
bonification had only just commenced. The differentiation of the inner xylem 
progressed mainly centripetally as the lignification of the outer xylem approached 
completion (fig. 23), and soon reached the stage represented in fig. 22. From this 
point it was a mere step to the fully differentiated state depicted in fig. 21. The 
general centripetal differentiation of the inner xylem and the ill-defined sequence 
in the outer cylinder were shown in all the materials examined. Although no 
well-defined stelar protoxylems were recognised, typical spiral protoxylems were 
occasionally observed in the departing leaf-traces. An example of this is represented 
in fig. 27. It shows an almost median and two intra-marginal protoxylem-groups 
which, if continued down as the leaf-trace-xylem becomes completely concurrent with 
