SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 647 
tissues of large rhizomes in figs. 12 and 13. In many specimens, as already 
mentioned, the epidermis has broken down, the surface thus losing the smooth 
appearance (fig. 29). 
The zone of outer cortex is usually well marked in the larger rhizomes, where 
it consists of some five or fewer layers of rather large cells which form a 
relatively persistent zone usually free from fungal hyphse (figs. 12 and 13). In the 
smaller and more root-like rhizomes the outer cortex is still clearly recognisable, 
but may be reduced to one or two layers. 
The inner cortex (figs. 13 and 29) is composed of narrower and more delicate 
cells. It is often more or less broken down, and frequently contains a particular 
fungus, which is usually wanting in the outer cortex on the one hand and the 
phloem on the other. The same features characterise this region in the small 
rhizomes, where it may be reduced to one or two layers of cells, and be difficult to 
distinguish from the phloem unless the fungus be present.* 
The phloem persists, and is fairly well preserved in many of the rhizomes. 
In the larger examples (fig. 4) it forms a broad, clear zone of closely associated 
elements, all of one kind. Even in the smallest rhizomes it is usually distinguish- 
able (fig. 14). The elements composing it, which are elongated tubes with pointed 
or transverse ends, will be described in the next section in connection with the 
transition region which has supplied the best-preserved examples (cf. figs. 
20 and 22). 
The xylem in rhizomes of all sizes forms a simple, more or less cylindrical, 
strand of tracheides, with no indication of a distinction between protoxylem and 
metaxylem. All the tracheides have the peculiar type of spiral thickening 
characteristic of Asteroxylon. This feature is shown in figs. 10 and 11, which are 
details from the rhizomes in figs. 8 and 6 respectively. It will be described in 
connection with the leafy stems. 
Transition Region from Rhizomes to Leafy Shoots. 
A series of intermediate stages has been traced between the characteristic 
structure of the rhizome, as described in the preceding section, and that of the 
leafy shoot of Asteroxylon. It has not as a rule been found possible to follow 
the changes in a series of sections of one specimen, but the intermediate grades 
are so completely represented as to leave no doubt as to the reality of the 
transition. 
The transition is indicated by a number of changes shown in different degrees 
by the various specimens. As compared with that of the rhizome, the epidermis 
becomes more distinct and definite, and has thicker external walls. The cortex, 
* The question as to the saprophytic or possible mycorhizal nature of this fungus is deferred to Part 1Y, in 
which the fungi present in the peat-bed will be considered. 
