96 



MISS KELLIE BANCEOFT ON 



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(6) The xylem elements have bordered pits arranged typically in two alternating 

 series. The pits are flattened and in contact. 



as in the Cordaitece and Ar 



conifers. 



(7) The medullary rays, both primary and secondary, are uniseriate. 



(8) The tissue accompanying the xylem-masses externally is much crushed, but 

 occasionally shows traces of thin-walled cells, radially arranged. It is considered as 

 representing phloem. 



III. AprriNiTiES AKD Kefeiience oe the Specime:?^. 



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In many respects the stem-fragment shows an approach to certain members af the 

 fossil group Medullosese, the structure of which is well known from the researches of 

 Goppert and Stenzel (2), Weber and Sterzel(i3), Solms-Laubach (11), Scott (7, 8, 9), and 

 other authors. The forms included under the genus Medullosa have been arranged by 

 Weber and Sterzel in form-cycles, each form-cycle being represented by a typical species 

 with varieties which may or may not be quite distinct. The principal form-cycles are 

 those of M. stellata, Cotta, Jf. porosa, Cotta, M. Solmsii, Schenk, and M. Zeuckarti, 

 Gopp. & Stenz. Evidently belonging to the same Medullosean family or plexus are 

 forms from different horizons and showing certain differences in structure and histology 

 from Medullosa itself. Such genera are Steloxylon^ Sutcliffia, and Colpoxylon, The 

 Cladoxylese are a distinct group showing certain points of resemblance to the Medul- 

 losese. The South African stem agrees most closely in general appearance, at least, with 

 M. Solmsii. The characters of this form-cycle may be briefly summarised as follows 

 (see 13, p. 73; Taf. iii. figs. 1-5; Taf. viii. fig. 3) : — Jf. Solmsii shows typically two 

 series of plate-rings, each ring representing a stele, and appearing to consist of a 

 normally orientated and an inversely orientated mass of secondary tissue, the lateral 

 portions of the ring being often poorly developed (see 13, Taf. iii. fig. 5; Taf. viii. 

 fig. 3). In variety typica the normal and inverse parts of the external rings are equally 

 strongly developed ; var. incrassata has the normal part most strongly deveh)ped ; and, 

 lastly, in var. lignosa there are several zones of secondary wood and bast outside the 

 two zones of plate-rings. In many cases the '* partial pith" containing primary 

 tracheides, and typically occupying the central part of a ring, is practically absent (13, 

 Taf. iii. fig. 5 ; Taf. viii. fig. 8). It may have been originally insignificant in amount, 

 or it may have become crushed dui'ing the life of the plant or during fossilisation (13, 

 p. 74). It seems to have entirely escaped preservation in the cases where a distinct 

 space is seen in the interior of the ring. Medullosa Solmsii, according to these 

 authors, has the closest xylem-structure known in the genus Medullosa (13, p. 114); 

 nothing seems to be actually known of the type of pitting of the tracheides in 

 M. Solmsiij though in the genus, as a whole, multiseriate bordered pits are the rule. 

 Phloem is not recognisable; the dark-coloured tissue surrounding the steles is 

 almost structureless. In the interior of the stem M. Solmsii shows very small 

 star-rings, and in the cortex also are similar structures arising from the inner series 

 of plate-rings. 



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