SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 615 
make it convenient to use the name columella for the sterile projection in Hornea . 
The cavity is filled by the numerous spores with cuticularised walls. 
The sporangia evidently arose by the transformation of the tips of certain 
branches of the plant. When the apex was simple, a single sporangium (such as that 
shown in fig. 58 to the right, and in fig. 64) resulted. When, however, the apex was 
in a more or less advanced stage of division, this is reflected in the subdivision of 
the sporangium and the lobing or branching of the columella, as shown in fig. 58 
to the left, and in fig. 63. 
The stalk of the sporangium is thus simply an ordinary branch of the plant. It 
shows the same structure as the stem, with epidermis, cortex, and stele more or less 
well preserved. Stalks bearing sporangia and showing the position of the stele are 
represented in 'figs. 58 and 60. The size of the sporangium differs according to the 
size of the stem bearing it. Thus the sporangium in fig. 58 is about 2 mm. long 
by slightly over 1 mm. broad, while that in fig. 64 is about 2 mm. in diameter. 
The wall of the sporangium was of considerable thickness (about '25 mm.), and 
its tissues, like those of the stems, are usually imperfectly preserved. An epidermal 
layer, a middle zone consisting of a number of layers of thin-walled cells, and a more 
resistant tapetal layer can be distinguished (figs. 65 and 67), as in the case of 
the sporangium of Rhynia. The wall shows irregularities in outline or projecting 
processes which do not seem to be wholly accounted for by accidental contraction. 
A common appearance is the flattening or broadening of the tip of the sporangium, 
as shown in figs. 58 and 59. 
The epidermal layer of the sporangium is, at least in some cases, more marked 
than that of the vegetative . stems. It is bounded by a cuticle, and has the outer 
walls of the cells thickened, the thickening extending inwards on the lateral walls. 
No indication of any place of dehiscence has been seen even in complete transverse 
sections. The structure of the epidermis is seen in figs. 67 and 68, while that of the 
wall as a whole is well shown in fig. 65. Below the epidermis comes a zone of some 
six layers of small thin-walled cells ; this is usually badly preserved. The innermost 
layer of the wall is the persistent “ tapetum,” which is continuous over the columella 
and thus forms a complete lining to the sporangial cavity. The tapetal cells are 
well shown in fig. 70. Their walls are dark, and give the impression of having been 
rigid, as in the case of the corresponding layer in the sporangium of Rhynia. No 
indication of tracheidal thickening of these cells has been met with. 
The columella (fig. 69) is composed of narrow, elongated, thin-walled cells which 
give the tissue, as usually preserved, a peculiar fibrous appearance. Though corre- 
sponding in position to a continuation of the stele of the stalk, it exhibits no 
agreement in histological structure with the central region of this, but resembles 
rather the phloem of the stele. The specimen shown in fig. 62 shows that the 
columella is directly continuous with the phloem of the stalk. 
The spores are often met with still associated in tetrads (fig. 7 1 ), while in other 
