728 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
The following conclusions as to the form of the leaf may be drawn from our observa- 
tions :— 
1. The leaf of Lyginodendron was a highly compound one, the branches of the 
rachis being given off alternately. 
2. The form and venation of the leaflets were those of a Sphenopteris.* 
3. Structure of the Petiole and Rachis. 
This again is a subject which has been so fully dealt with in previous Memoirs, 
that: only a few points need be discussed here. 
None of the preparations hitherto figured afford conclusive evidence as to the 
structure of the vascular bundle. We will, therefore, first call attention to Plate 18, 
photograph 5, which shows a petiole in transverse section; from its small size—2 millims. 
in maximum diameter—it no doubt represents one of the ramifications of the main 
petiole. The preservation is remarkably perfect, in fact, scarcely a cell is lost. The con- 
centric structure of the bundle comes out with astonishing clearness, which could not 
be exceeded in a preparation from a recent Fern. The small celled, thin-walled tissue 
constituting the phloém is absolutely perfect and completely surrounds the V-shaped 
xylem. The maximum thickness of the phloém-zone is found on the convex or 
morphologically lower side of the bundle ; it thins out somewhat at the lateral angles, 
and again attains a considerable thickness on the concave upper face. The larger 
thin-walled cells bordering on the cortex are best regarded as pericycle. Neither 
here nor in any other specimen, do we find any differentiated bundle-sheath or 
epidermis ; considering the perfection of the preservation we are probably justified in 
concluding that in Lyginodendron, as in the Marattiacee of the present day, a 
specialized endodermis was not developed. Secretory sacs are scattered among the 
phloém-elements. 
Every well-preserved transverse section of a petiole shows the same fact, that the 
bundle was concentric Where two bundles are present, as usually happens in main 
petioles, and sometimes in their secondary branches, each bundle is surrounded by its 
own zone of phloém, except where the two are on the point of fusing (cf. Memoir 
XVII., Plate 13, fig. 2). Longitudinal sections confirm these conclusions.t 
The position of the spiral tracheides or protoxylem in the petiolar bundle is not 
always easy to determine in the transverse sections, but with the help of the longi- 
tudinal sections this can be done. There were always several such groups. Thus, 
in the specimen shown in photograph 5, there were certainly three, one near the 
* The following are the most important specimens which throw light on the form of the leaf :—O.N. 
134, 135, 137, and 139; these four form a series ; 143,147; 1191-1198, a series; 1855, 1856, 1885p, 1979. 
t See especially C.N. 1978 and 1985; from the latter, fig. 13 was drawn. Both these sections are 
from petioles found in actual continuity with the stems of Lyginodendron. For additional good trans- 
verse sections of petioles, cf. C.N, 139, 146, 1194, 1854, 1857, 1984, 
