838 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
inwards towards the centre. The next section, represented in fig. 26 and more highly 
magnified in fig. 27, passed through the tip of the stem below the growing point ; it 
was so close to the latter that the widening of the stem leads to the superficial tissues 
being cut somewhat tangentially. Epidermis (ep.), outer cortex (o.c.), and the outer 
zone of the inner cortex ( i.c.o .) can be distinguished in order, although the cortical 
tissues are not completely differentiated. The condition of the middle trabecular 
zone of the inner cortex ( i.c.m .) is of interest. The trabecular structure is clearly 
marked, but the radial depth of the zone is short. The inner zone of the inner 
cortex ( i.c.i .) is of full width, and its rather imperfectly preserved tissue passes 
without a sharp limit into the stele. 
In the stele the xylem (cc.) forming the forked poles of what would become the 
stellate .xylem is alone differentiated, there being no tracheides developed in the 
central connecting portion. Leaf- traces (l.t.) departing from the poles of the stele are 
seen around the latter. The soft tissues of the stele are preserved, though somewhat 
imperfectly. It is clear from this section that the poles of the stelar xylem were 
differentiated close behind the apical meristem. 
The other sections of this series show that the central portion of the stelar xylem 
did not become differentiated for some distance behind the apex. A stele from the 
middle of the series is represented in fig. 28, the lowest section of the series is shown 
complete in fig. 29 and its stele more highly magnified in fig. 30. This lowest 
section shows that at the base of the series the differentiation of the stelar xylem 
was hardly further advanced than in figs. 26 and 27, although a few elements of the 
connecting metaxylem can be recognised. The outer cortex has attained the 
structure found in mature stems ; the tissues of the inner cortex are too poorly 
preserved and crushed to be instructive. 
This complete series confirms and extends the information as to the arrangement 
of the leaves in the bud shown in Part III, PI. V, fig. 36, and as to the structure of 
the stele with its xylem imperfectly differentiated shown in Part III, PI. VIII, 
figs. 59, 60. The series affords conclusive evidence that the general differentiation 
of the stellate xylem of the shoots of Asteroxylon was centripetal ; the position of 
the protoxylem in the ends of the rays was shown in Part III to be as a rule enclosed 
or mesarch. 
Some Bearings of the Rhynie Plants on the General Morphology of the 
PTERIDOPHYTA AND THE ORIGIN OF THE ORGANISATION OF LAND-PLANTS. 
The deposit at Rhynie gives us a remarkable glimpse of the vegetation of the 
Early Devonian * period. The four higher plants, which are all Vascular Cryptogams, 
have the double interest of being the most simply organised Pteridophy ta known 
and of being the most ancient fully-known forms of this group. In relation to the 
* The term Early Devonian is here used to include the period represented by the Lower and Middle Old Red 
Sandstone, hut to exclude the Upper Old Red Sandstone. 
