SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 661 
cf 106, 118). There may be a small island of thin-walled phloem (fig. 109) or a 
larger island (fig. 1 1 0) within the strand of xylem. This internal phloem may be 
continuous with the outer phloem, making the cross section of the xylem horse-shoe 
shaped (fig. 111). Larger examples of this distribution of the tissues are also met with 
(fig. 114). Where the phloem extends right across, the two xylem strands are more 
or less distinctly and widely separate and embedded in a common mass of phloem 
(figs. 112, 113, 107). This leads on to two distinct steles lying within the same 
cortex (figs. 115, 116). 
The range of structure in these specimens, though they cannot be regarded as 
constituting a series, is evidently consistent with a preparation for dichotomous 
division of the axis. The more or less completely double steles are, however, so 
frequent as compared with evidence of actual branching, that this may not be the 
complete explanation. The specimens so far described show no departing lateral traces. 
Other examples have been met with, though less abundantly, in which the xylem 
of the stele was stellate or triangular in transverse section, and in which small traces 
were departing from the angles (figs. 117 and 119). These are more like small twigs 
of Asteroxylon , and it is noteworthy that in them the tracheides tend to be some- 
what wider than is usual in these peculiar axes. This is well shown in fig. 119. 
In yet other examples the stele was of the usual type for these axes, but was 
splitting off small lateral portions (figs. 120, 126 at a). It is of interest that these 
specimens were usually in the immediate neighbourhood of the sporangia, to be 
described below (fig. 126). 
While the general relations of the tissues in these axes is satisfactorily ascertained, 
the poor preservation makes any detailed description of the tissues difficult. In only 
a few cases, such as the specimen illustrated in figs. 106 and 118, were all the tissues, 
including the cortex, fairly well preserved. 
The epidermis is smooth, and its cells have their outer walls distinctly thickened 
and covered with a cuticle. As mentioned above, this layer often remains when the 
internal tissues have decayed (fig. 114). Stomata with short guard cells and a small 
pore have been observed in the epidermis (fig. 121). The cortex in the examples in 
which it is best preserved (fig. 118) is a uniform tissue composed of parenchymatous 
cells. Little can be said of the phloem except that it consisted of narrow, elongated, 
thin-walled elements. These, as preserved, tend to show in transverse section small 
dark triangular markings at the junction of the cells similar to those to which 
attention was directed in the corresponding tissue of Hornea Lignieri. 
The tracheides composing the xylem were narrow, elongated tubes, with pointed 
ends and brown walls (fig. 122). Though they remain connected and show the out- 
line clearly in transverse and longitudinal sections, repeated examination of numerous 
examples has failed to show the type of thickening of the tracheide walls. There was 
almost certainly a thickening that has been lost or altered by decay, but we are not 
prepared to place weight on such indications of its nature as we have yet observed, 
