49 ° Gwy line- Vaughan . — Observations on the . 
the thickened end of the xylem strand it is accompanied by the soft tissues 
that line the concavity of the trace. Sometimes these are represented by 
a few layers of the xylem sheath lying on theadaxial side of the protoxylem 
and forming a bay between it and the centripetal metaxylem. In more 
advanced cases the phloem and pericycle also behave in the same way. 
Accordingly just below the departure of a trace the xylem strand is more 
or less strongly bulged outwards on that side (Photos 15 and 17). The 
protoxylem divides as before, and at about the same time the curve of the 
xylem breaks across on its adaxial side a short distance from its extremity 
(Photos 16 and 18). The small mass of xylem thus left behind gradually 
becomes joined on to the rest of the xylem strand, usually a little to the 
outside of the inner protoxylem, so that the latter continues to be endarch. 
More rarely the adaxial xylem joins on immediately in front of the inner 
protoxylem, so that this becomes temporarily mesarch. 
This adaxial mass of xylem can only be regarded as representing the 
centripetal xylem in the previously described method of branching. It is 
interesting to note that in some cases it also may become very scanty, 
being reduced to one or two tracheae only (Photo 19), thus furnishing 
another method of transition to the simplest type of branching first 
described. 
The further development of the method of branching is related in quite 
a simple manner to the increase in size of the parent trace and of the trace 
of the branch. As the latter becomes larger the bay becomes wider and its 
central region becomes invaded by the endodermis and the central ground 
tissue of the parent trace, so that the departure of the branch-trace leaves 
a gap in the parent trace communicating directly with the external ground 
tissue. This is the case in Todea barbara , Todea superba , Osmunda bipinnata , 
and Osmunda javanica . If the branch-trace is curved into a ring at the 
point of attachment, as in Osmunda regatis , it opens out a short distance 
above. In Todea hymenophylloide s Osmunda regatis var. palustris , and 
Osmnnda regatis var. japonic a the parent trace is not interrupted even at its 
largest branchings. 
The position in the leaf at which any particular type of pinna-trace 
departure is to be found varies greatly from one plant to another and even 
from leaf to leaf, and seems to depend a good deal upon the stature and 
vigour of the leaf. 
The simplified or reduced type of branching can arise on the one hand 
by the loss of the centripetal xylem on the adaxial side of a mesarch 
protoxylem immersed in a solid mass of xylem, or on the other hand by loss 
of the xylem mass on the adaxial side of a bay of parenchyma. Since the 
method of branching with a bay of parenchyma is derived from that with 
a solid thick-ended xylem strand the two methods are connected and the 
missing adaxial xylem is the same in the two cases. 
