Vascular System of Matonia pectinata . 485 
not die out, and eventually enters into connexion with the dilatation at the 
base of the next trace. The apex of the rhizome in this specimen had died 
and its tissues partly perished. A young leaf with undifferentiated tissue 
and showing its circinate vernation is attached to this portion of the stem. 
Plant D (Fig. 4), as will be seen, bears leaves of a type no more complex 
than those of C, nor does its rather bulkier stele show any definite advance 
in structure. The transverse section of the proximal end shows essentially 
the same type that we saw in the distal end of C, i.e. the central phloem of the 
stele encloses both internal endodermal cells and also a free strand of xylem. 
But here the xylem-strand is comparatively bulky and centrally situated, 
while the endodermal cells are few and more or less scattered (Plate XXXII, 
Fig. 15 ). Presently the endodermal cells unite to form a strand one or two cells 
thick and crescent-shaped in cross-section on one side of the central xylem, 
while the latter comes into connexion with the external xylem, probably in 
the mid-dorsal line. A leaf-trace is now gradually given off on one side of 
this connexion. The strand of endodermal cells in the concavity of the 
leaf-trace is perhaps 1 unconnected with the internal stelar endodermis, but 
the corresponding phloem is certainly connected both with internal and 
external stelar phloem. The leaf-trace, after it becomes free from the stele, 
is concentric in structure. It is kidney-shaped in section and its wings con- 
tain large scalariform elements, while its centre is composed of a strand 
of small tracheids. After the departure of the leaf-trace, there is a gap 
left in the xylem-ring 2 , and this is closed by the external xylem of the 
stele, the inner xylem-strand becoming immediately again detached from 
the outer ring. Later it again enters into connexion with the outer ring, 
and the next trace arises from the other side of the point of connexion. 
Though rather bulkier, it arises in identically the same way and has 
evidently the same structure as the last. The endodermis of its concavity 
is certainly connected both with the internal and with the external stelar 
endodermis. Before the trace is fully detached, the tissues of the stele cease 
to be fully differentiated, and the stele shortly becomes promiscuously 
invaded by cells like those of the inner cortex, the rest of the stelar cells 
having meristematic characters. The stele is then lost in a uniform mass 
of brown cells, while a curved knob of meristematic cells, representing the 
actual growing point, is found attached to the ventral surface. The explana- 
tion of this appearance is probably due to a checking in this particular 
plant of apical growth beyond the point of insertion of the last leaf, and the 
consequent passing over of the majority of the undifferentiated cells into 
a passive condition without differentiation. 
1 Owing to defects in the series this point could not be absolutely determined. 
2 Where such a gap occurs it may be called a * xylem leaf-gap,’ or a ‘ xylem-gap/ as distinguished 
from a true leaf-gap in which the tissue of the pith is put in connexion with that of the cortex owing 
to an interruption in all the stelar tissues. 
