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PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
representing the same structure in the fossil and living Cryptogams) demonstrates 
that we must not expect to find in the primaeval types exactly the same genetic pheno- 
mena as those with which we are familiar at the present day, when the differentiation of 
Cryptogams from Phanerogams has progressed so far by the degradation of the former and 
the elevation of the latter types. When no true Dicotyledons existed, their places being 
taken by an arborescent type of Cryptogams, the differences to which I have called 
attention prepare us for recognizing without surprise the possibility of other genetic 
distinctions, such as we find in the exogenous growth so generally characteristic of the 
Carboniferous plants. 
Description of the Plates. 
PLATE XLI. 
Fig. 1. Transverse section of a young Lepiclodendroid twig, enlarged 20 diameters. 
Fig. 2. Transverse section of the tip of a yet younger twig, enlarged 24 diameters. 
Fig. 3. Transverse section of the central vascular cylinder of fig. 2, enlarged 54 
diameters. 
Fig. 4. Transverse section of the central vascular cylinder of fig. 1, enlarged 54 
diameters. 
Fig. 5. Longitudinal section of a similar twig to fig. 1, enlarged 50 diameters. 
Figs. 6, 7. Padial sections of the bark of specimens like fig. 1, enlarged 50 diameters. 
Fig. 8. Transverse section of the vascular cylinder of a branch larger than fig. 1, 
enlarged 54 diameters. 
PLATE XLII. 
Fig. 9. Transverse section of a young branch which has reached the Diploxylon form, 
enlarged 24 diameters. 
Fig. 10. Eadial section of a portion of the vascular medullary cylinder and ligneous 
zone of fig. 9, enlarged 55 diameters. 
Fig. 11. Transverse section of the central axis of a large Diploxyloid stem, natural size. 
Fig. 12. Tangential section of part of the ligneous zone of fig. 11, enlarged 10 diameters. 
Fig. 13. Tangential section of part of the ligneous zone of fig. 11, yet further enlarged 
32 diameters. 
Fig. 14. Vertical section of the medulla and vascular medullary cylinder of a Diploxyloid 
stem, enlarged 12 diameters. 
Fig. 15. Transverse section of a portion of fig. 14, exhibiting the junction of the vessels (c) 
of the cylinder with the cells (b) of the pith, enlarged 50 diameters. 
Fig. 15 a . Kaclial section of the outermost layers of the bark with leaf-petioles attached, 
natural size. 
