98 Dr. W. C. Williamson on 



point at the periphery of the primary tracheal strand in 

 the form of a very small number of barred tracheae, to 

 the periphery of which additional ones are steadily added, 

 as well as on each side of the first formed ones, thus 

 producing a small crescentic group, thickest radially 

 in its centre. This crescent grows as it commenced, 

 radially and at each slender horn, until it forms a 

 perfect ring enclosing the primary tracheal cylinder. 

 After this the growths are added to the entire periphery, 

 thus producing an exogenous zone, what in my largest perfect 

 specimen (C.N. 1922D), attains to a thickness of nearly '2 

 of an inch ; but in another larger fragment (C.N. 362AA) 

 reaches '55. The section 1922D, though the periphery of 

 its prosenchymatous zone has disappeared along with the 

 more superficial cortical tissues, has a mean diameter of four 

 inches, what remains of its prosenchymatous zone alone 

 having a thickness of i"3. 



Axial Structures. 



Medulla. 



B.— p. 199, Fig. la. 



L.-p. 285, Fig. I. C.N. 335, Fig. 3, C.N. 339. Fig. 5, C.N. 343. 

 Fig. 6, C.N. 347. 



Composition of Medullary Centres. 



B.— p. 199, Fig. 3. Fig. 4. See C.N. 356 and 360. 

 L. — p. 285, Figs. 3 and 5, C.N. 339, 343 and 376A. See longitudinal 

 section, 376A. 



Primary Tracheal Strand. 



The Etui Medullaire of Brotigniart. This is the Primary Xylem of the 

 Memoirs. 



B. — p. 199 and 201, Figs. 3 and 4. 



L.— p. 286, Fig. 3, C.N. 339. See also C.N. 367. 

 Seconday Xylem Strand.* 



B. — p. 199, Fig. le. 



L.— p. 286 et seq. Fig. 6h, C.N. 347. Fig. 7h, C.N. 363. ' 

 Crescentic Development. 



L.— p. 286, et seq. Fig. 3, C.N. 339. Fig. 4, C.N. 343, Fig. 6, C.N. 

 347. Fig. 7, C.N. 363. 



*This is the Secondary or exogenous zone of the Xylem. I have introduced this new 

 nomenclature for the first time into the Memoir, Part XIX. (BB) for the purpose of bringing 

 our nomenclature into unison with that much used on the Continent. 



