614 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
intercellular spaces were present there (figs. 42, 43, 50, 51). In the more decayed 
stems the phloem may or may not be recognisable. It seems to have been less 
characteristically developed at the lower part of the stele near its insertion into the 
rhizome. All that can be said regarding the phloem in longitudinal section is that 
it consisted of narrower and more elongated elements than the cortex. 
The xylem formed a centrally situated, solid strand of tracheides. It exhibits a 
considerable range in diameter, probably in relation to the regions of the stem-system 
of the plant. A xylem strand of moderate size is shown in transverse section in 
fig. 47, and others of larger size at the same magnification in figs. 45 and 48, with 
which fig. 46 can be compared. The xylem is shown more highly magnified in 
figs. 50 and 51. A distinction between narrower central, and wider peripheral 
tracheides is a constant feature. This holds for the steles of moderate diameter near 
to the rhizome, for the steles of large diameter, which we presume came from a region 
between this and the finer branches, and for the finer branches themselves. There are 
no thin-walled cells mixed with the tracheides, but the central xylem often appears 
broken down within a ring of peripheral xylem composed of wider and intact elements 
(figs. 43, 45, 46, 48). When such steles are cut longitudinally this appearance is 
seen to be due to repeated transverse breaks or interruptions of the core of central 
xylem, the tracheides of the peripheral xylem remaining continuous (fig. 49). 
The thickenings on the tracheide walls have in most cases disappeared owing to 
the decayed condition of the stems. They are often best shown in the region of the 
stele near to the rhizome. In favourable specimens there is a distinct thickening 
of narrow bands forming irregularly connected rings or a spiral (figs. 52 and 53). 
As the extreme base of the stele in the rhizome is reached, the zone of phloem 
disappears and the tracheides become shorter and wider (fig. 54) and pass gradually 
into the brown-celled tissue at the base of the stele. 
The stems of Hornea branched dichotomously, and sometimes, show the stele 
dividing (figs. 55 and 56), or two equal steles enclosed in the same cortex (figs. 42 
and 57). From the frequency with which it is met with in smaller stems, this 
dichotomous branching was probably most marked in the upper region of the plant. 
Sporangium. (Plates IX-X.) 
Sporangia of a remarkable type occur associated with the remains of the vegetative 
organs of Hornea, especially with the finer branching stems. The proof that they 
belong to this plant is afforded by sporangia terminating stems with the structure 
described above (figs. 58, 60, 61). 
The general construction of the sporangium will be evident from the longitudinal 
sections in figs. 58-60, and the transverse sections in figs. 63 and 64. As these 
show, the sporangial cavity was enclosed by a fairly thick wall, and had a sterile 
column of tissue projecting so far from its base that the actual cavity is dome-shaped. 
The resemblance to the columella of some bryophytic sporogonia is so great as to 
