SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 655 
The large, thin- walled elements of the phloem were preserved in the small stele 
represented in fig. 50. The large stele in fig. 72 is the best example that has been 
met with. In this the phloem is complete, both in the bays and around the 
stele. No clear distinction can be drawn between the thin- walled tissue in these 
two regions. The elements in the centre of the bays are wider than those abutting 
on the xylem, and suggest a possible distinction of phloem proper and conjunctive 
parenchyma. It does not appear advisable, however, to attempt such distinctions 
of tissues on the specimens so far examined. The elements of the phloem with 
dark contents in fig. 72 have probably been cut across not far from the transverse 
end-wall. 
The phloem in the stem is as a rule so much more decayed than in the rhizomes 
or transition region that it is better to rely on these ( of fig. 22) for showing the 
structure in longitudinal section. An example from a stem is represented in fig. 70. 
The elements of phloem within the cortex are elongated and compressed. The wider 
elements from one of the bays are partially decayed. From this and other specimens 
it was clear that they were wide elongated tubes, which sometimes had the almost 
transverse end-walls to which attention has been directed in the case of the better- 
preserved phloem of the transition region. 
It is convenient to select for the more detailed description of the xylem and of 
the structure of the tracheides examples in which the protoxylem was recognisable, 
but the fact that this was not always possible must be borne in mind. This is the 
case, for example, with the steles in figs. 50, 53, and 57, which may be contrasted with 
fig. 56, where the position of the immersed protoxylems comes out prominently. 
Figs. 74, 75, and 76 represent three rays of the xylem of one large stele. There 
is here no difficulty in distinguishing from the larger tracheides making up the bulk 
of the xylem the groups of small and compressed tracheides ( px .) within the ends of 
the rays but close to the surface. These small tracheides are the protoxylem, and 
they are here enclosed by the larger elements of the metaxylem. Some- 
times a protoxylem. group appears divided (fig. 75), and when a division is extending 
to the surface there is the preparation for the separation of a leaf-trace. The 
tracheides in figs. 74 to 76 are to some extent separated by the decay of the middle 
lamella. The apparently uniform thickening of the wall proves on careful examination 
to be the optical expression of a thin wall from which the localised thickening projects 
inwards, thus diminishing the cavity. There are none of the characteristics of a pitted 
wall in the transverse sections of the tracheides, either of the protoxylem or metaxylem. 
The character of the thickening is evident when the tracheides are seen in longi- 
tudinal section (figs. 77 to 79). The wall of the tracheide is moderately thin, but 
has a spiral or irregularly spiral thickening. This projects into the cavity as rather thin 
flat ledges, which, when seen from above in the transverse section, gives the appearance 
of a uniformly thickened wall. The tracheides of the protoxylem (figs. 77 and 78, 
px.) are much narrower, and have spiral or annular thickening. The clear distinction 
of protoxylem and metaxylem, while the thickening of all the tracheides is spiral, is 
