700 
PEOFESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
Description op the Plates. 
Pachiopteris aspera. 
Plate LI. fig. 1. Transverse section of a matured petiole, magnified 16 diameters. 
„ fig. 2. Longitudinal section of fig. 1, magnified 20 diameters. 
„ fig. 3. Vessels of vascular bundle: A, reticulate ; B, barred; C, spiral. 
„ fig. 4. Two lacunae of the bark, from a vertical section, magnified 65 
diameters. 
„ fig. 5. Tangential section of the outermost layer of the bark, magnified 
80 diameters. 
Plate LII. fig. 6. Transverse section of the middle portion of a petiole, magnified 16 
diameters. 
„ fig. 7. Transverse section of upper part of a petiole, with the gutter-shaped 
vascular bundle, magnified 16 diameters. 
„ fig. 8. Transverse section of the petiole yet nearer to the tip, the vascular 
bundle becoming circular, and the abortive hairs conspicuous, 
magnified 16 diameters. 
„ fig. 9. Transverse section of the petiole near its extremity, with small 
cylindrical bundle, magnified 16 diameters. 
„ fig. 10. Transverse section of the extreme termination of a petiole, 
magnified 16 diameters. 
,, fig. 11. Longitudinal section of a young petiole, with transverse layers of 
dense cells, enlarged 12 diameters. 
„ fig. 12. Portion of a section like fig. 11, magnified 30 diameters. 
„ fig. 13. Longitudinal section of the upper part of a petiole, with secondary 
pinnules and leaflets, magnified 16 diameters. 
Plate LIII. fig. 14. Longitudinal section of a secondary pinnule, magnified 16 
diameters. 
Pecopteris. 
Plate LII. fig. 15. Transverse section of a leaflet of a Pecopteris , magnified 20 
diameters. 
Pachiopteris aspera. 
„ fig. 16. Section of a leaflet or leaflets of Bachiopteris aspera (direction 
doubtful), magnified 20 diameters. 
Pecopteris. 
Plate LIII. fig. 17. Longitudinal and transverse section of part of a leaflet of a 
Pecopteris , magnified 20 diameters. 
„ fig. 18. Obliquely longitudinal section of a leaflet [of a Pecopteris , mag- 
nified 16 diameters. 
fig. 19. Leaflet of a Pecopteris, enlarged 20 diameters. 
