. Bulletin of the British Museum (Natural History), Geology. MORPHOLOGY AND RELATIONSHIPS OF RACHIOPTERIS CYLINDRICA 57 and situated somewhat nearer the adaxial than the abaxial side (PL 8, fig. 1). This stage is well shown in Williamson Colin, slide 187 and is figured by Seward (1910, text-fig. 305B) and by Scott (1920, text-fig. 156A). Exceptionally the proto- xylem group supplying the branch does not divide into two and occupies a median position in the branch stele. An example of this kind is figured diagrammatically Ky Fig. 1. Psalixochlesna cylindrica (Will.) Camera lucida outline
. Bulletin of the British Museum (Natural History), Geology. MORPHOLOGY AND RELATIONSHIPS OF RACHIOPTERIS CYLINDRICA 57 and situated somewhat nearer the adaxial than the abaxial side (PL 8, fig. 1). This stage is well shown in Williamson Colin, slide 187 and is figured by Seward (1910, text-fig. 305B) and by Scott (1920, text-fig. 156A). Exceptionally the proto- xylem group supplying the branch does not divide into two and occupies a median position in the branch stele. An example of this kind is figured diagrammatically Ky Fig. 1. Psalixochlesna cylindrica (Will.) Camera lucida outline of an obliquely longi- tudinal section passing through the stem apex, st, stem stele giving off petiole bundle, pb, and root, r : r1, root cut transversely passing through the stem cortex : dev, develop- ing petiole showing circinate vernation: h, dotted outline indicating extent of the hair sheath, x 12. (Holden Coll., no. 611). by Bancroft (1915, text-fig. 1). There is some variation in the diameter of the tracheids forming the metaxylem of the branch. As a general rule these tracheids have the same dimensions as those of the parent stem but there are a number of examples ( Holden Colin. 301a) in which they are appreciably smaller in diameter and the differentiation into small central and larger peripheral tracheids is not evident. Petiole : The initial stages in the emission of the petiole trace bear a strong resemblance to those leading to the giving off of a branch. The assumption of the ovoid appearance of the stem stele in transverse section and the positioning of one. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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