SHOWING STRUCTURE, FROM THE RHYNIE CHERT BCD, ABERDEENSHIRE. 675 
demonstrate the passage of the one type to the other, and perhaps a record of this 
was hardly to be looked for. 
In Rliynia and Hornea we have revealed to us a much simpler type of Vascular 
Cryptogam than any with which we were previously acquainted. This type suggests 
the convergence of Pteridophyta and Bryophyta backwards to an Algal stock. 
The knowledge of Asteroxylon confirms and enriches our conception of a more 
complex but archaic type of the Vascular Cryptogams which supports the idea of the 
divergence of the great classes of Pteridophyta from a common type, and links this 
on to the simpler Rhyniaceae. 
EXPLANATION OF PLATES. 
(All the figures are from untouched photographs.) 
Asteroxylon Maclciei, Kidston and Lang. 
Plate I. 
Fig. 1. General view of a vertical section from the upper part of bed A" 1. Large stems of Asteroxylon 
are enclosed in the dark sandy matrix above. The clear peat below is composed of Hornea, but contains 
rhizomes of Asteroxylon. x 2§ . (No. 2461.) 
Fig. 2. Similar section from another block of the silicified peat from the same bed. In the sandy matrix 
above are stems of Asteroxylon, and at (a) a large rhizome containing a smaller intrusive one. Into the peat 
below, made up of Hornea and Rhynia major, rhizomes of Asteroxylon extend vertically ; the One at ( b ) 
perforates a rhizome of Hornea. x 2. (No. 2462.) 
Fig. 3. Small rhizomes in relation to large stems of Asteroxylon enclosed in the sandy matrix, a, rhizome 
cut transversely in the cortex of the large stem ; b, rhizome cut longitudinally as it lay in the peat and 
passed through another stem of Asteroxylon. x 14. (No. 2463.) 
Fig. 4. Large rhizome in transverse section into which another rhizome of considerable size has intruded, 
while the cortex of this in turn encloses two small rhizomes, ox., outer cortex of the large rhizome ; i.c., 
inner cortex ; ph., phloem; x., xylem. x 14. (No. 2464.) 
Fig. 5. Transverse section of rhizome of moderate size showing division of stele preparatory to dichotomous 
branching, x 14. (No. 2465.) 
Fig. 6. Slightly oblique longitudinal section of a rhizome of moderate size, ep., epidermis ; o.c., outer 
cortex; i c., inner cortex; ph., phloem; x., xylem. x 14. (No. 2466.) 
Fig. 7. Partially decayed specimen of rhizome in obliquely longitudinal section showing dichotomous 
branching. x 14. (No. 2467.) 
Plate II. 
Fig. 8. Portion of the small rhizome in longitudinal section shown in fig. 3. x 33. (No. 2463.) 
Fig. 9. The small rhizome in transverse section shown in fig. 3. x 33. (No. 2463.) 
Fig. 10. Portion of the xylem of the small rhizome in figs. 3 and 8 showing the spirally thickened 
tracheides. x 210. (No. 2463.) 
Fig. 11. Portion of the xylem of the rhizome in fig. 6. The imperfectly preserved phloem is seen to the 
right, x 210. (No. 2466.) 
Fig. 12. Portion of a longitudinal section of the large rhizome in fig. 6 showing the superficial tissues 
well preserved, ep., epidermis; o.c., outer cortex ; i.c., inner cortex. x 105. (No. 2466.) 
Fig. 13. Portion of the outer tissues of another large rhizome in longitudinal section, ep., epidermis; 
o.c., outer cortex; i.c., inner cortex, x 60. (No. 2468.) 
Fig. 14. Two small root-like rhizomes in transverse section. x 33. (No. 2469.) 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 26). 
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