264 
PROEESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
bundle, h. Smaller divergent petiole, or root, containing one of the secondary 
vascular bundles. 
Fig. 13. Central axial vascular cylinder. Enlarged 18| diameters. 
a. Medulla composed of cells intermingled with small vessels, b. Seven 
clusters of vessels blending peripherally into an almost continuous vascular 
zone, g . Bundle-sheath composed of small vertically elongated cells. 
Fig. 14. Longitudinal section of the axial vascular cylinder. 
a. Medulla. b,b. Vascular cylinder, e. Cells of the cortical parenchyma. 
g. Bundle-sheath. Enlarged 18^ diameters. 
Fig. 15. Transverse section of vessels ( b ) of the axial cylinder exhibiting the tylose 
cells (i) with which each vessel is filled. Enlarged 100 diameters. 
Fig. 16. Longitudinal section of a similar vessel. 
i. Tylose cells. Enlarged 100 diameters. 
Fig. 17. The primary vascular bundle, fig. 4, c. 
g. The bundle, c. Inner bark. g". The bundle-sheath. 
Fig. 18. Transverse section of a detached petiole. 
c. Central vascular bundle, e. Inner cortex, f. Outer cortex. Enlarged 
7-| diameters. 
Fig. 19. The portion ( b V) of the vascular cylinder of fig. 6. 
b', b'. A cluster of vessels preparing to separate themselves as a primary 
vascular bundle. Enlarged 18|- diameters. 
Fig. 20. Similarly enlarged representation of the projection b' from the central cylinder 
of fig. 7. Enlarged 18-j diameters. 
a. Medullary cells, b. Cluster of axial vessels, b', b". Vessels separating 
from the axial cylinder b. d. Secondary bundle, e. Cells of inner cortex. 
Enlarged 18-|- diameters. 
Fig. 21. Oblique longitudinal section of part of a central vascular axis. 
b. Vessels of the axial cylinder, c. A primary bundle departing from the 
axial cylinder, d. A secondary bundle, e. Parenchyma of the inner bark. 
f. Outer bark. g. Bundle-sheath. Enlarged 6 diameters. 
Fig. 22. Cells and vessels from fig. 19, b'. Enlarged 62 diameters. 
Fig. 23. Transverse section of the primary petiolar bundle, fig. 5, c. Enlarged 18-^ 
diameters. 
d. A secondary bundle preparing to branch off from the primary one. 
Fig. 24. Transverse section of the secondary bundle, fig. 6, d', now becoming detached 
as a small petiole or root and invested by its cortical tissues. Enlarged 18-^- 
diameters. 
Fig. 25. Section of a sporangium of a Fern. Enlarged 75 diameters. 
a. Annulus, b. Peduncle, c. Spores. 
Fig. 26. Single spore of fig. 25, enlarged 320 diameters. 
