166 Mr Seward, Notes on the Binney Collection 



have passed through the trace at a lower level of its downward 

 course than in the case of the leaf-trace It of fig. 5, PL v., which 

 is just entering the zone of secondary wood. The greater separa- 

 tion of the protoxylem groups — px, px — and the relatively greater 

 abundance of parenchyma in the leaf-trace It shown in fig. 3, PI. v., 

 are features characteristic of a trace as seen in its gradual passage 

 into the metaxylem of the stele. The more internal elements of a 

 leaf-trace are the first to be replaced by shorter and broader 

 tracheids and to place themselves into direct contact with the 

 larger tracheal elements of the metaxylem ; this is illustrated by 

 text-figures 3 and 4, which represent a longitudinal section of a 

 leaf-trace seen at about that part of its course indicated by It 1 in 

 fig. 1, PI. V. In text-fig. 3 the outer leaf- trace elements It retain 

 their elongated narrow form, but on the side next the metaxylem, 

 m, the tracheids are shorter, broader and more irregular, passing 

 into direct connexion with the metaxylem elements. 



Section I. 10 shows very clearly the behaviour of the tissue 

 elements of a leaf-trace as it passes down the stem ; the innermost 

 tracheids first pass gradually into shorter and rather broader 

 tracheids, and these are succeeded by still broader and shorter and 

 less regular tracheids, which are often more or less strongly curved, 

 and finally these elements become linked on to the short elements 

 of the metaxylem with which the leaf-trace tissue is gradually 

 merged. These changes are accompanied by the gradual spreading 

 of the leaf-trace tissue in a radial direction — as shown in fig. 1, 

 PL v., It, It 1. The shorter and broader tracheids which succeed 

 the normal longer and narrower xylem elements of a leaf-trace in 

 its downward course, are curiously irregular in width, being swollen 

 or contracted as well as curved, as shown in the central part of 

 text-figure 3. 



At the opposite side of the primary xylem to that on which 

 the leaf-trace It, It 1 is seen in fig. 1 there are a few long and broad 

 irregular and rather twisted tracheids between the edge of the 

 secondary xylem and the typical short or flattened metaxylem 

 tracheids; these represent the last indications of a leaf- trace 

 which has descended deep enough in the stem to almost entirely 

 lose its individuality. 



In the section reproduced in text-figure 4 the leaf-trace has 

 almost died out, it is shown at a lower level than in text- 

 figure 3 ; the leaf-trace tracheids are fewer in number, and are 

 represented by a few comparatively broad and rather irregular 

 elements. 



It would appear that each leaf-trace passes through approxi- 

 mately four internodes of the stem before entirely losing its 

 individuality; in the deeper part of its course it is represented 

 only by a few elongated reticulately pitted and narrower spirally 



