A KEW MEDULLOSEAN STEM. 



99 



fragment is yery small (2'4 cm. X2cm. diam. and 1'3 cm. long), only four steles and part 

 of two others being represented, it is impossible to determine the original arrangement 



of vascular structures. If anything may be judged from the position of those present, 

 however, it would seem that they were more or less radially arranged — that is, the 

 narrow partial pith lay in a radial direction. The steles are almost complete rings ; only 

 at one pole of the partial pith is the continuity of the secondary growth broken, and at 

 this point, in the case of two of the steles, is attached a peculiar wing-like mass of 

 secondary elements, probably representing a trace. In one instance, the wing is less 

 closely in contact with the main stele and has diTided into two parts (12, Taf. iii. 

 fig. 5). At the other pole of the stele, secondary growth shows its greatest development. 

 The partial pith is narrow and, like the phloem, usually not preserved : in one case its 

 cellular nature may be determined (12, Taf. iii. fig. 11). The steles pursue a fairly 

 straight course, so far as can be determined from the fragment. The wood is compact, 

 with uniseriate rays separating groups of from two or three to eight or nine rows of 

 tracheides (12, Taf. iii. fig. 11). In this there is agreement with the South African 

 fossil. The chief feature of interest in C. Kidstoni, from the point of view of comparison 

 of the two types, lies in the pitting of the tracheides. The radial walls possess typically 

 a single series of elongate-elliptical bordered pits, giving a scalariform effect 

 (cf. character of the group Oladoxylese). Here and there, however, often in the same 

 tracheide with the single series, a double row of bordered pits occurs (12, Taf. iii. 

 fig- 7) *, the pits being either alternate or opposite — they are always flattened and in 

 contact. In the case of alternate pitting the appearance is very similar to that presented 

 by the tracheides of the South African fossil. This cannot, however, be regarded as 

 indicating any relationship between the two types, which show points of dissimilarity in 

 other respects, such as stelar organisation. C. Kidstoni is interesting, in that the pitting 

 of its tracheides points to a transition from a simpler more fern-like type to that 

 exhibited by the South African fossil, Cordaitese, and Araucariese. The Cladoxyleai 

 need not be considered so far as the closer relationships of the new type are concerned, 

 but must be mentioned with the Medulloseae as representative of the plexus of polystelic 

 types to which the stem under consideration is apparently referable. 



With regard to the wider affinities of the new specimen, it is possible that the biseriate 

 Araucarian type of pitting exhibited by its xylem elements indicates a point of contact 

 more or less remote with the ancestral forms of the Araucarieae. The combination of 

 Araucarian pitting of the xylem elements with the Medullosean arrangement of steles is 

 interesting, and the possibility is suggested that there may have existed other simpler 

 or even monostelic forms, as yet unknown, also exhibiting Araucarian pitting, and 

 that from these or their ancestral stock the Araucariese may have been derived either 

 directly or indirectly. It is extremely unsafe to base any conclusions upon the pitting 

 of the xylem elements, for how far such a character may serve as a criterion in the 

 consideration of relationship between types is not as yet thoroughly understood, and 

 the above suggestion is made with due regard to this fact. It is, however, interesting 



* 



In Solms-Laubacli's figures tlie bordering of the pita is not shown. 



