1039 
A ffinities of Sutcliffia . 
which in turn break up completely into leaf-trace bundles. Further 
evidence in favour of this conclusion is provided by the behaviour of the 
other c meristeles’. 1 
The course of the ‘ meristele ’ f3 will next be traced ; in the lowest 
section of the series (see Text-fig. 2 ) it is already entirely free from the main 
stele M ; it lies on the side of the stele remote from the ‘ meristeles ’ a and a 1 
which have just been described. It is entirely though somewhat unevenly 
surrounded by secondary tissues, and contains two groups of primary 
tracheides. In IV the ‘ meristele ’ is preparing to divide into two parts, 
each containing one primary xylem group ; this division is completed in V 
(see also PL XCI, Fig. 1 ). One of these two parts, /3 1 , shows no secondary 
tissues for some considerable distance on the side remote from /3 2 (Text- 
fig. 8) ; this is probably due almost entirely to injury in this region. /3 1 shows 
no further change until XXVIII, when a portion of the primary wood at its 
upper end shows signs of detachment ; this separation is completed by 
XLI, and the rounded mass of primary xylem /3 3 has passed out from the 
secondary wood. It cannot be traced above this level, so that its exact 
significance cannot be stated definitely. From its size, and the position of 
the protoxylem elements, /3 3 probably represents a radially symmetrical 
leaf-trace strand such as was indicated as occurring during the breaking up 
of the ‘ meristele * a. Returning to the other half of the divided ‘ meristele ’ 
(3, namely (3 2 (Text- fig. 8, Fig. V), almost immediately after its separation 
from (3 1 the secondary wood on the outer side opens out, exposing the 
primary xylem. This ‘ opening out ’ of the ‘ meristele ’ is undoubtedly 
partly the result of injury, but there can be little doubt that some move- 
ment of the vascular mass took place in life, in order to prepare for the 
fusion with the main stele M, which occurs immediately above this level. 
The course of events is shown in Text-fig. 8, Figs. II-XX ; comparison with 
PI. XCI, Fig. 1 , shows that the portion of the stele M with which /3 2 finally 
fuses has been considerably injured, though how far the absence of secondary 
wood at this part of M is a natural feature it is difficult to say — in all 
probability a break in the secondary tissues of M occurred preparatory 
to the entrance of /3 2 , thus allowing for the fusion of the secondary and 
primary tissues of the former with the corresponding ones of the latter. 
During the process of fusion, a small arc of secondary wood with two 
or three primary tracheides attached to its margin became isolated. The 
ultimate fate of this arc could not, however, be determined. This instance 
of fusion of a ‘ meristele ’ with the stele, the only one occurring in the 
series, is of considerable interest ; it is unparalleled in S. insignis , for Scott 
decided that what appeared to be a case of re-fusion in that stem was due to 
1 The ultimate breaking up of the ‘ meristeles ’ a and a 1 is not shown in Text-figs. 4 
and 5. The model ends with Section LIV, for the last four sections were too fragmentary to 
include. 
