SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED. ABERDEENSHIRE. 649 
to these, small leaf-traces are given off from the angles of the xylem. A scale- 
leaf from another specimen is more highly magnified in fig. 23. 
In the stem represented in fig. 24 the stele has all the features to be described 
below for the large leafy stems, but the cortex is of the simpler type, and the leaves 
appear to have been small and scale-like. The large specimen on PI. XIII, fig. 95, 
is better preserved, but its construction is at about the same grade of complexity. 
In the details of its histology the transition region is intermediate between the 
rhizomes and the leafy shoots, but more like the former. Only special features, 
or those tissues that are particularly well shown in this region, need be mentioned. 
The tissues are brought out most clearly in fig. 20, which is a transverse section 
of an early transition region from the same axis as fig. 17 ; in fig. 22, which is a 
portion of the longitudinal section in fig. 16, and is also of the early transition 
region; and in fig. 21, which is a portion of the late transition region shown in 
fig. 19. 
The epidermis is usually well preserved, and its small cells covered with cuticle 
form a more definite boundary than on the rhizome (fig. 20, ep.). Stomata were 
present in it (fig. 73). The outer and inner cortex resemble the corresponding 
tissues of the large rhizomes, and, as fig. 20 shows, may be very well preserved; 
The inner cortex is often infested with the same fungus as that occurring in the 
rhizomes (figs. 20 and 22). 
The xylem calls for no special description, except to note that when it becomes 
stellate in the more advanced transition region the rays from which the leaf-traces 
depart are composed, in some cases at least, of narrower tracheides. 
The phloem has tended to persist, and some of the best-preserved examples 
of this tissue are supplied by the transition region. It consists of closely associated 
elements, all of one kind (figs. 20 and 21). In longitudinal section (fig. 22) these 
appear as elongated tubular cells, which sometimes fit together with pointed ends, 
and at other times have transverse end-walls. 
In the less advanced specimens of the transition region the phloem, as in the 
rhizome, forms a broad, clear zone around the more or less cylindrical xylem. As 
the latter becomes stellate its projecting rays divide up the zone of phloem, but do 
not reach to the periphery of this. There is thus phloem between the rays of xylem 
and also forming a narrow zone outside the rays (fig. 21). The xylem of a leaf-trace 
passing out to the scale-leaves is surrounded by a sheath of this outer zone of phloem 
(fig. 21, It.) 
The Leafy Shoot. 
Leafy shoots ranging in diameter from 1 centimetre to 1 millimetre ( cf. figs. 96 
and 30) form the most abundant material of Asteroxylon met with in those portions 
of the chert that have been examined. The structure in the large as in the small 
specimens is primary, and was determined once for all in development. The 
