358 
PROFESSOR W. WILLIAMSON ON THE ORGANIZATION 
Fig. 3. A small portion of fig. 2, enlarged 44 diameters, showing the junction of the 
medullary cells, a, with the innermost vessels, b. 
Fig. 4. Obliquely transverse section of a specimen like fig. 2. a. Medullary cells of 
stem intersected transversely, a. Medullary cells of branch intersected 
almost longitudinally. 
Fig. 5. Transverse section of a stem with an unusually large medulla, a, and 
numerous primary vascular wedges, b, resembling those of some Catamites, 
but devoid of any longitudinal internodal canals, x, x. Intruded Stig- 
marian rootlets. Magnified G diameters. 
Fig. 6. Tangential section of a part of a primary vascular zone exhibiting the 
vascular laminae, (6) separated by primary (c) and secondary (c) 
medullary rays. 
Fig. 7. Transverse section of a stem in which the bark (d) is preserved, and also 
exhibiting the bundle of enlarged vessels occupying the medullary angle 
of each primary vascular wedge. 
Calamites. 
Fig. 8. Transverse section of a very young Calamite enlarged 26 diameters. 
b. Medulla, e. Longitudinal internodal canals, h. Cortex. 
Fig. 9. Similar section to fig. 8, but deprived of its cortex. Enlarged 2G diameters. 
Fig. 10. Similar section to fig. 8, but somewhat more advanced in development. 
a. A fissure in the medulla indicating the commencing formation of a 
fistular medulla. Enlarged 26 diameters. 
Fig. 1 1. Similar section to fig. 10, but decorticated, and with the fistular medullary 
canals (e) ; further enlarged, b. Medulla. 
PLATE 20. 
Fig. 12. Transverse section of a young decorticated stem in which also the fistular 
medullary cavity, a, is yet more enlarged than in fig. 10. Enlarged 
26 diameters. c. Primary medullary rays. e. Internodal canals. 
f Vascular wedges. 
Fig. 13. Transverse section of a larger stem in which the vascular wedges (f) are 
in the same stage of development as in fig. 12, but in which the fistular 
medullary cavity (a) is larger, h. Cortex. Magnified 26 diameters. 
Fig. 14. Transverse section of a large Calamite. a. Fistular medullary cavity. 
b. Thin layer of medullary cells, f Primary vascular wedges merging 
into a continuous vascular structure towards their peripheral extremities. 
h. Thick, shattered layer of bark chiefly composed of prismatic pro- 
senchyma. Two-thirds natural size. 
