458 ME. W. C. WOESDELL ON THE COMPAEATIVE 



My final conclusion, then, is as follows : — The vascular tissues of the stems of existing' 

 Cycads have been derived, with modifications corresponding to the period which has 

 intervened, from the vascular tissues of the stems of the Medullosese or plants with 

 closely -allied structure, and along the following lines : the central cylinder of our 

 modem plants is the direct derivative of the primary ring of concentric strands or steles 

 of the fossil forms, while the succeeding secondary vascular rings of the three Cycadean 

 genera which possess them, and the secondary concentric or partially concentric strands, 

 or parts of such, which occur in the cortex of these three plants, have had their origin in 

 the successive outer rings of the Medulloseae, which (although this point has not yet 

 been definitely ascertained) will probably be eventually proved to be also secondary in 

 origin ; for secondary structures can only derive descent from secondary structures and 

 not from primary. Therefore all the secondary vascular tissues of modern Cycadean 

 stems constitute part of the inherent morphological structure, and in this respect differ 

 completely from, and should never be compared with, the secondary extrafascicular 

 structures of sucli plants as the Sapindacese, Tecoma^ and Beta^ which, as already 

 stated above, are purely adaptive in character and have been assumed for purposes of 

 nutrition. 



In conclusion, I have to thank the authorities of the Royal Gardens, Kew, for so- 

 kindly furnishing me with much of the valuable material for this investigation. 



EXPLANATION OF PLATE XLIII. 



The following are the abbreviations used : — 



^>A = phloem ; na?' = nomial tracheides of the secondary wood; 7?a;'*=reticulately -thickened tracheides 

 of the secondary wood; joa^=protoxylera ; /6=leaf-base; //6 = leaf- trace bundle ; c/ = cortex ; ^ = girdles; 

 /) = pith; »i6=medullary bundles; a? = xylem; 7wr = medullary ray; i?r^ = first vascular ring; i;r'' = second 

 vascular ring ; vr' = third vascular ring; vr'= fourth vascular ring; mc = mucilage-canal ; c*^ cs\ c*', 

 cs' = cortical strands representing the successive vascular rings. 



Encephalartos caffer, Lehm. 



Fig. 1. Radial section of a girdle-leaf-trace bundle, x 130. 



Fig. 3. Diagram of a radial section, showing a connecting strand between two vascular rings. 



Fig. 4. Transverse section of a large medullary bundle from the lower region of the stem, x 35. 



Fig. 5. Transverse section of a young medullary bundle and mucilage-canal. X 130. 



Fig. 7. Tangential section of the xylem of the first vascular ring, showing a medullary bundle and 



mucilage-canal in the medullary ray. x 35. 

 Fig. 8. Diagram of a radial section of the first vascular ring, showing the connection with the xylem of 



a medullary and a leaf-trace bundle. 



