SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 613 
section was just touching the edge of the base of a stem-stele. The' stems attached 
to the protocormous rhizomes in most of the specimens figured show their steles at 
some point or other. 
The characteristic base of the vascular strand of a stem springing from a rhizome 
is best seen when the section is truly vertical to the rhizome and follows the median 
plane of the stem (figs. 35-38). The best specimen is that shown in figs. 35 and 36, 
the other two specimens serving to confirm it. As fig. 35 shows, one lobe of a 
rhizome has been cut vertically, and the section follows in an accurately longitudinal 
direction the stele of the stem arising from the lobe ; the cortex has decayed from 
the further part of the stele as it lies in the peat. The rhizome shows the structure 
already described, and bore rhizoids below. While there is no vascular tissue in the 
rhizome itself, the stele of the stem when traced downwards into the upper part 
of the rhizome (figs. 35, 36) ends in connection with a brown-celled tissue without 
tracheidal thickenings. The inverted cup-shape of this mass of tissue is characteristic, 
though not often so favourably shown as in this specimen ; it is seen also in figs. 37 
and 38. It explains the frequent occurrence of patches of brown tissue in sections 
of rhizomes (cf figs. 29 and 31). 
Stem. (Plates VII and VIII.) 
The basal regions of the stems attached to the rhizomes described above were 
often more or less decayed, but were sometimes well preserved. Portions of both 
the upper and lower regions of similar stems in various stages of decay make up the 
peat. A group of the best-preserved specimens is shown in fig. 39, some being cut 
in transverse and one in longitudinal section. The general similarity in structure 
to the stem of Rhynia will be evident, though the preservation is not so good in the 
material of Hornea yet examined. The well-defined cuticle and thickened epidermal 
wall, the broad cortex, and the stele with a zone of phloem surrounding the rather 
stout central strand of xylem, are distinguishable in fig. 39. The transverse section 
of a small stem from the upper region of the plant in fig. 40 may be compared with 
this. The corresponding regions are shown in longitudinal section in fig. 41. 
The tissues may now be described in greater detail, starting from the outside 
(figs. 40 and 4l). There was a well-marked cuticle. The epidermis had its outer 
walls somewhat thickened, but otherwise did not differ much from the underlying 
cortical cells. No stomata have been observed, but owing to the state of preservation 
of most of the material much weight cannot be attached to this negative result. 
The cortical tissues cannot be sharply distinguished into outer and inner cortex, 
though the cells diminish in diameter on passing inwards. The cortical cells are 
somewhat elongated, and have transverse end walls (fig. 41). 
Within the cortex and immediately around the xylem was a zone of thin-walled 
elements which readily collapsed. This zone represents the phloem (figs. 42 and 43). 
It often has a peculiar appearance, as if the cells were thickened at the angles or small 
