Affinities of Sutcliffia. 
1053 
VI. The Extrafascicular Strands. 
wp s 
One of the most interesting features in the vascular anatomy of the 
new stem is the presence of numerous extrafascicular strands around the 
vascular system proper. In life it is probable that these strands formed 
a complete anastomosing network around the steles and ‘ meristeles ’, for in 
the fossil they extend round three sides ; their absence on the fourth is 
accounted for by the fact that considerable injury occurred on this side 
in the course of petrifaction. The strands appear to cluster at the ends of 
the long diameter of the stem, but this appearance is perhaps partly 
exaggerated by the lateral pressure 
to which the stem has been sub- 
jected (Text-figs. 3 and 4). The 
arrangement of these extrafasci- 
cular strands varies considerably ; 
usually they take the form of norm- 
ally orientated bands of secondary 
wood and bast, frequently they 
assume a fan-shaped appearance, 
incompletely concentric structures 
may occur, rarely nearly perfectly 
concentric bundles are fotmd, while 
in one case a band-like strand 
showed inverted orientation of the 
vascular tissues. The size as well 
as the arrangement of these strands 
shows great diversity (Text-fig. 2 
and PI. XCI, Fig. 1). In every 
case, no matter what the size, 
arrangement, or orientation of the 
strands, there are found on the 
side of the wood remote from the 
phloem a number of short isodiametric tracheides (Text-fig. 19). 
These short broad tracheides have, as a rule, reticulate markings 
on their walls, though occasionally pitted elements seem to be present. 
Their average maximum diameter is about 100 pt ; their length, which 
is slightly greater, averages 66-6 fx. They are clearly marked off from the 
long, multiseriately pitted tracheides which compose the bulk of the wood 
of the extrafascicular strands, for the diameter of these elements only 
reaches about 58 /*, although in some strands the dimensions are greater 
and may reach 115*8 /x ; 1 it is very difficult to determine clearly the short 
Text-fig. 19. Transverse section of part of 
an extrafascicular strand, showing isodiametric 
tracheides ( t.t .) on its inner margin, xy 1 2 = second- 
ary xylem. x 84. 
1 They are thus somewhat smaller than the secondary tracheides of the stele and ‘ meristeles 
the average diameter in the latter case being 1 59 /a. 
