160 MEDULLOSEAE [CH. 



An abnormal seedling of Araucaria Bidwillii described by Shaw 1 

 exhibits features analogous to those in some cycadean seedlings : 

 within the normal stele a cambium forms an inversely orientated 

 vascular cylinder which at a lower level becomes continuous with 

 the outer centrifugal tissue, the whole vascular system being 

 eventually represented by two concentric steles. The polystelic 

 stage is a development of a monostelic condition, and the inner 

 or inverted portion of each of the two concentric steles is derived 

 from an inversely orientated cylinder in the central region of the 

 root. This abnormal root does not, however, supply an argument 

 in favour of the derivation of a monostelic type of stem from one 

 that was polystelic, but it shows a close relation between the two 

 plans in one organ. The seedling is not altogether normal in 

 form apart from structure and it is not improbable that the ana- 

 tomical abnormality is connected with some pathological cause. 



It has been suggested 2 that Worsdell attaches too much 

 phylogenetic significance to the irregularities in the disposition 

 and form of the vascular bundles in the peduncle of Stangeria, 

 and the criticism that insufficient allowance is made for the 

 possible reaction on structure of the special physiological require- 

 ments of reproductive shoots is well founded. Granting an 

 overestimate of the arguments drawn from the occasional occur- 

 rence of concentric vascular strands, a considerable body of 

 evidence remains in favour of Worsdell's main contention. 



Mrs Thoday 3 has drawn attention to certain features exhibited 

 by the inflorescence-axes of Welwitschia, particularly the occur- 

 rence of concentric and inversely orientated bundles, similar to 

 those characteristic of the seedling of Cycas siamensis^ described 

 by Matte, and to anatomical characters occasionally present in 

 adult cycadean stems and normally represented in Medullosa. 

 She is of opinion that the occurrence in Welwitschia of certain 

 Medullosan features has a phylogenetic significance. The dif- 

 ferences between the Gnetales, Medulloseae, and Cycadales are 

 considerable, and it would seem unlikely that the anatomical 

 resemblances described by Mrs Thoday are of great value as criteria 

 of close relationship. The comparison of Lagenostoma with 



1 Shaw (09). 2 de Fraine (12) p. 1060. 



3 Sykes, M. G. (10 2 ); (10). 4 Matte (04) Pis. xv. xvi. 



