49 
Zygopteris Grayi of Williamson, 
xylem and protoxylem of the stem ; the two distal branches pass into 
the peripheral loops ; the proximal branch takes a median position and 
ultimately supplies the axillary shoot. Thus the whole organization of 
the leaf-trace is dependent on the internal xylem of the stem, for it is 
from this that its first-formed elements are supplied. 
Structure of the Axillary Stele. 
As we have seen, the axillary stele is represented in the undivided 
trace by the median adaxial bulge with its central strand of small 
tracheides and parenchyma (PI. I, Phot. 8 ; PI. Ill, Figs, i and 2). The 
bulge becomes more marked as the trace passes further out (PI. I, Phot. 6 ; 
PI. Ill, Fig. 2). 
At the beginning of the series of transverse sections, the axillary stele 
seen far out in the cortex is just separating from the Jeaf-trace, 1 and its 
outline is already clearly defined, the line of separation passing through 
the thick abaxial xylem (PI. I, Phot. 1, a.s). It has a somewhat elliptical 
central strand at this level. The same stele at a rather higher level, after 
its complete separation, is shown in PI. IV, Fig. 9. Allowing for slight 
damage, the stele is approximately circular in section ; the central strand 
(‘ mixed pith ’) has a somewhat angular outline, possibly an indication of 
its ultimate stellate form. The internal xylem appears to form a solid, 
rather irregular mass, and has not assumed the annular arrangement 
characteristic of the main stele. The position of the protoxylem is not 
easy to determine ; some of the smallest elements lie on the outside of 
the internal xylem, but it is not certain that they all do. 
The longitudinal section (PL IV, Fig. 10) shows the structure of 
another axillary stele, no doubt that belonging to the next lower leaf- 
trace. The broad outer zone of scalariform tracheides encloses the strand 
of small elements forming the internal xylem, and in one part the spiral 
tracheides of the protoxylem (px) are evident. They appear to be ad- 
jacent to the parenchyma of the central strand, and thus to lie on the 
outside of the internal xylem. 
The axillary stele, in this part of its course, thus shows the stelar 
structure of the plant in its simplest form. The essential features — peri- 
pheral and internal xylem-systems — are present, but none of the compli- 
cations of form and tissue-arrangement belonging to the fully developed 
stele have yet appeared. 
Some further points connected with the axillary stele will be con- 
sidered when we return to the Williamson specimens (see p. 54). 
1 This is the trace corresponding to arm 1 of the stem-stele. 
E 
