XXI] 



ANATOMY 



139 



are followed to earlier horizons. But it will be seen that, in a general way though not 

 always in consecutive detail, the trend is the reverse of this. Six salient stages of stelar 

 complexity are involved in the series as recognised by Kidston and Gwynne-Vaughan. 

 They may be seriated from the simplest to the most complex, thus : 



(i) A solid homogeneous xylem {J>roiostele). 



This is seen in the basal region of all young sporelings, but it is not described for any 

 living species in the adult stage. Certain Botryopterideae, however, and especially 

 Grammatopteris from the Permian of Autun, were regarded by Kidston and Gwynne- 

 Vaughan as possessing structure primitively Osmundaceous {I.e. No. i, p. 778). It was 

 thought by Dr Stopes that a certain cretaceous fossil from Queensland, named Osinuttdites 

 Kidsto?ii, showed this structure in the adult together with secondary thickening, as in 

 Botrychioxylon {Atm. of Bot. xxxv, p. 55, 1921). But re-examination of the specimen 

 has yielded a quite different interpretation (Posthumus, Ann. of Bot. xxxviii, p. 215, 1924). 

 ^cv en 2Jh ^yx 



Fig. 430. Thainuoptcns S(/'iucIiteiid(ilii Eich. Part of a stele. 

 /er=pericycle; t;/ = endodermis; //i= phloem; /x=proto- 

 xylem. a: = outerxylem; ({' = innerxylem. PermianAge. (After 

 Kidston and Gwynne-Vaughan. From Seward.) ( x 13.) 



The incident is mentioned here so as to correct the statements made in Vol. i, pp. 129, 

 137, which were based on Dr Stopes' paper. 



(2) A solid pithless xylem., but heterogeneous in structure. 



This appears in two species of Zalesskya from the Upper Permian of Russia: but the 

 characteristic structure is particularly well preserved in Thamnopteris Schlechtendalii 

 from the same source. The general construction of the shoot corresponds remarkably with 

 that of modern Osmundaceae : but the stele is simpler (Fig. 429). There are two zones of 

 xylem which are seen to be quite distinct structurally (Fig. 430). The protoxylems are 

 sunk in the continuous zone of outer xylem, while the leaf-traces separate from its outer 

 surface in the protostelic fashion, expanding as already described into the characteristic 

 Osmundaceous curve (Vol. I, Fig. 155). 



