Anatomy of the Ophioglossaceae . III. 27 
The stele of the second branch had from its base more central 
parenchyma than that of the first specimen, and even before the departure 
of the first leaf-trace may be said to have a definite pith. This is clearly 
marked above the leaf-trace departure, and it may be noted that nothing 
in the arrangement of the cells suggests ‘ intrusive ’ origin of the pith, which 
is indeed enclosed within the peripherally placed remains of the inner 
xylem. A short distance further on (cf. Text-fig. 5) the first root springs 
from the ventral side of the branch, and from that level onwards the stele 
is much larger, and has a wide pith surrounded by a tube of xylem. There 
is little indication of inner xylem in this region, the pith occupying its 
position, though occasional tracheides occur. 
The distal portion of the basal region of this branch, as seen in longi¬ 
tudinal section, naturally corresponds to the appearance of the next portion 
cut transversely. In PI. Ill, Photo 36, the first leaf-trace ( l.t : . 1 ) is seen just 
after it has undergone division. The endodermis is complete around the 
stele, which shows the relatively large pith surrounded by a tube of xylem. 
This is almost entirely outer (centripetal) xylem, though occasional single 
tracheides of the inner xylem were found. Dorsally, the xylem for the 
second leaf-trace is recognizable in the stelar ring, its protoxylem, in the 
absence of inner xylem, abutting on the pith. The next photographs 
(PI. Ill, Photos 37-39) show the interesting histological changes associated 
with the departure of this leaf-trace. Photo 37 shows the endarch xylem 
of the trace just separated from that of the stele. The gap in the tube of 
outer xylem, which is left by its separation, is partially bridged across by 
the development of a number of tracheides of inner xylem. In Photo 38 
the development of inner xylem across the gap in the outer xylem is much 
more marked, and inner xylem is also present in the lower part of the stele. 
The leaf-trace is still within the common endodermis. In the next stage 
(Photo 39) the trace and stele have their respective endodermal layers 
complete, the trace shows only slight indications of adaxial extension of its 
xylem, and the stele has almost returned to the condition from which we 
started (cf. Photos 39 and 36), in having almost no tracheides of inner 
xylem developed in the central parenchyma or pith. The position of the 
protoxylem of the nascent third leaf-trace is recognizable at px? The 
attachment of the second root is seen at the ventral side of the stele in 
Photo 38. 
The rhythm exhibited in the disappearance and reappearance of the 
inner xylem in this small medullated stele will be discussed later, after 
the corresponding facts for young plants have been described. The two 
branches described show agreement in essentials of construction, though 
medullation by replacement of the inner xylem was earlier and better 
marked in one case than in the other. Both show, however, that the small 
stele is to be regarded as mesarch, although the inner xylem consists 
