1037 
Affinities of Sutcliffia. 
may be remembered that in Scott’s fossil the structure of the ‘ meristeles * 
was similar to that of the main stele, and, like it, sometimes showed a slight 
indication of the beginning of secondary growth. 
III. Course of the ‘Meristeles’. 
In order to elucidate the behaviour of the ‘ meristeles ’ as completely 
as possible, the course of three strands, designated a, /?, and y, will be traced. 
In Text-fig. 2, taken from the lowest section of the series, the main stele 
(m) has a large hook-shaped projection at its left-hand side (a ) ; it consists 
almost entirely of secondary wood, but embedded within it are two groups 
of primary xylem (Text-fig. 8, Fig. II) ; these groups are respectively 
labelled a and a 1 . 
In III 1 a slight indentation appears at the extreme left of this pro- 
jection ; it occurs immediately opposite a deep incision present on the 
opposite side of the mass, and lies just above one of the groups of primary 
wood ; the end of the hook is evidently preparing for detachment. In IV 
the process is almost entirely complete, while in V a 1 is quite independent 
(Text-fig. 8, Figs. IV and V, and PI. XCI, Fig. i). Very rapidly, secondary 
wood almost, but not quite, encloses the detached end of a 1 , while a soon 
shows no sign that disturbance has taken place. No further change occurs 
in a 1 — it never becomes entirely surrounded by a zone of secondary wood— 
until Section XXXIX, 3! inches higher up (9-5 cm.). 
At this level its primary wood is beginning to divide into two portions, 
a division which is completed by XLVI. It is difficult to say with certainty 
what is happening in this upper part of the stem, for the tissues have been 
subjected to considerable crushing and the preservation is poor ; but in 
Sections XLV to XLIX, though the * meristele ’ a 1 appears to be fusing 
with a, the appearance is in all probability deceptive and due to mechanical 
pressure ; 2 moreover, a 1 is dividing up into ultimate leaf-trace strands. 
Returning to the ‘ meristele ’ a, in II a group of primary xylem elements 
can be traced passing from the stele M to join the isolated cluster already 
present ; in V it has passed nearly through the secondary wood ofM and the 
‘meristele’ is detaching itself from the main stele (PI. XCI, Fig. 1) ; it 
becomes entirely free in VIII (Text-fig. 8, Figs. II-VIII). As soon as these 
changes are completed the secondary wood immediately closes round 
it, and also round M ; this condition is maintained for a distance of 
3J inches (8*2 cm.) (Text-figs. 4 and 5). From Section XXXVIII upwards 
the sequence of events cannot be given with absolute certainty owing to the 
crushing which has taken place, but the primary xylem of a undoubtedly 
divides into two parts ; the smaller portion is passing out through the 
1 The Roman numerals refer to the number of the slide. 
2 In the model this strand is necessarily represented as fusing with a (Text-fig. 4), 
