XXXII] RHETINANGIUM 181 



RHETINANGIEAE. 



Khetinangium. Gordon. 

 Rhetinangium Arberi Gordon. 



The stem on which this genus is founded was discovered by Dr 

 Gordon 1 in the Calcif erous Sandstone series of Pettycur : a specimen 

 collected by Dr Kidston in Berwickshire may be specifically 

 identical with the Pettycur plant. We know nothing of the 

 leaves or reproductive organs of Rhetinangium. The stem, 

 approximately 2 cm. in diameter, was probably cylindrical ; it 

 possesses a single stele consisting mainly of a central primary 

 region occupied by anastomosing groups of tracheids, 130 150//, 

 in diameter, embedded in parenchyma containing numerous 

 secretory sacs and ducts. In the peripheral region of the stele 

 the groups of tracheids consist of narrower elements characterised 

 by exarch protoxylem. Each peripheral group forms the base of 

 a wedge of secondary xylem (fig. 450, x 2 ), the primary medullary 

 rays being in direct connexion with some of the parenchyma of 

 the primary xylem. The secondary tracheids, 45 85/u, in diameter, 

 have multiseriate bordered pits on their radial walls and the 

 rays are broad and deep as in Heterangium and Lyginopteris. 

 With the exception of the external position of the protoxylem, 

 the stele of Rhetinangium is practically identical with that of 

 Heterangium, though in Rhetinangium the primary tracheids form 

 larger and fewer groups. The inner cortex is composed of thin- 

 walled cells with many secretory sacs: there are no stereome 

 elements. In the outer cortex (fig. 449, cr) radially disposed 

 bands of stereome form a reticulum with narrow and very long 

 meshes like that of Medullosean petioles. 



The recently recorded occurrence of polydesmic petioles 2 in 

 Heterangium is of special interest from the point of view of the 

 comparison of that genus with Rhetinangium and the Medulloseae. 



The very broad decurrent petiole-bases are a striking feature, 

 the major diameter of the pulvinus-like base of the leaf -stalk 

 exceeding that of the stem (fig. 449, p). Several xylem-strands 



1 Gordon (12). The rich development of secretory tissue suggested the name 

 Rhetinangium (prjTLvt], resinous gum): the same prefix had previously been 

 adopted by Renault in his genus Retinodendron [Renault (96) A. p. 365]. 



2 Scott (15). 



