Leaf anatomy of the South African Danthonieae ( Poaceae ) . XVIII . Centropodia mossamedensis

The leaf blade anatomy of Centropodia mossamedensis (Rendle) T. A. Cope [= Asthenatherum mossamedense (Rendle) Conert] is described and illustrated. This description is based on freshly fixed material and confirms that this species has Kranz anatomy with the C4 photosynthetic pathway. The anatomy differs little from that of C. glauca and both undoubtedly belong to the same genus which is justifiably separated from the other danthonoid genera.


INTRODUCTION
In a previous paper (Ellis 1984) in this series the ana tomy of Centropodia mossamedensis (Rendle) T. A.
Cope [= Asthenatherum mossamedense (Rendle) Conert] (Cope 1983) was briefly described.This descrip tion was based on herbarium material and the anatomical preparations were not of a very high quality.Subsequent to the above study, fresh material of C. mossamedensis was collected and fixed in the field, yielding good qual ity leaf blade transverse sections.The results are de scribed, illustrated and compared with the leaf anatomy of C. glauca (Nees) T. A. Cope [= Asthenatherum glaucum (Nees) Nevskij.

Plants of C. mossamedensis were collected in South
West Africa/Namibia.Herbarium voucher specimens were prepared for verification by the National Herbarium (PRE) where they are now housed.
Leaf blade segments were removed and immediately fixed in FAA.Leaf blade transverse sections and abaxial epidermal scrapes were prepared following the methods outlined in a previous paper in this series (Ellis 1988).
The standardized terminology of Ellis (1976Ellis ( , 1979) ) was used for the anatomical descriptions together with the following abbreviations:  metaxylem vessels narrow, with a diameter slightly less than that of the obs cells; diameter greater than that of the ibs cells.Vascular bundle sheaths: double; slightly ellip tical to almost rounded; both sheaths entire around all vbs (Figure 1C); no extensions although a few Kranz cells may be located outside the outer sheath (Figure 1C); parenchyma sheath cells very numerous (15-26), regular in size and shape, fan-shaped with straight radial walls and inflated outer tangential walls; specialized, large, centnpetally situated chloroplasts conspicuous; ibs complete around 1' and 3'vbs; cells with slight secon dary thickening.Sclerenchyma: small adaxial girders as sociated with all l 'vbs and strands with the 3'vbs; taper toward the bundles; similar abaxial girders and strands rated by short cells or stomata.Stomata: low dome shaped (Figure 1D & G); not in regular files and occur in most long cell files; 1 or 2 interstomatal cells between successive stomata in a file.Intercostal short cells: absent or very rare (Figure ID) or irregular (Figure 1G); no cork or silica cells but just very short epidermal cells.Papillae: absent.Prickles: costal, either very common (Figure ID & H) or rare (Figure 1G); barbs either well developed or virtually absent.Hooks: absent.Micro hairs: present on all specimens but rare (Figure 1H); both cells elongated but distal cell not tapering to a pointed apex; two cells about equal in length.Macrohairs: ab sent.Silica bodies: variable, irregular dumbbell-shaped (Figure 1G), short and narrow (Figure 1H) or horizon tally elongated rectangular (Figure ID); occur through out costal zones; sometimes associated with cork cells but often not.

DISCUSSION AND CONCLUSIONS
The transectional leaf anatomy compares very closely with that of C. glauca (Ellis 1984).Both are undoub tedly Kranz with radiate mesophyll, and no chloren chyma cells are more than one cell distant from a Kranz cell.This indicates the presence of the C4 photosynthetic pathway which is confirmed by a 12c/13c ratio of -1 2 ,6 °/00 (De Winter & Hardy 8021).The outer bundle sheath has a regular outline and is Kranz with centripetally lo cated specialized chloroplasts.This structure is typical of that characteristic of the NAD-me subtype of the C4 pho tosynthetic pathway but this has yet to be confirmed as Centropodia has not yet been biochemically typed (Hattersley 1987).
Anatomical differences between C. mossamedensis and C. glauca are only minor, particularly the leaf in transverse section.Vessel element diameter is proportio nally greater in C. mossamedensis where they are slightly wider than the inner bundle sheath cells but they are, nevertheless, still relatively narrow.The bulliform cells occupy less than half the leaf thickness in C. mossa medensis but in C. glauca they are equal to at least half the leaf thickness.In transection no elongated prickle hairs are evident as in many C. glauca specimens.
Superficially the abaxial epidermis differs consider ably from that of C. glauca.No interlocking prickles resembling macrohairs are present and the unique macro hairs with corrugated cell walls, as in C. glauca var.lasiophyllum, were not observed.These epidermal differences are visually very striking but it must be remembered that C. glauca exhibits conti nuous anatomical variation from those specimens with conspicuous interlocking prickle hairs to specimens wi thout this hair type (Ellis 1984).This variation pattern is associated with an ecological cline from the extremely arid Namib Desert eastward to the Kalahari.The anat omy of C. mossamedensis appears to be a northward expression of this cline along a moisture gradient and C. mossamedensis may merely represent a continuation of this reduction trend evident in C. glauca.
The two species are distinct morphologically (Conert 1962) and also appear to occupy different niches.C. glauca is a species of the loose red sands of the Kalahari dunes whereas C. mossamedensis is confined to gravelly or coarse waterborne sands in dry watercourses.Their separation at species level is, therefore, not questioned by this study even though these two species do not ex hibit significant leaf anatomical differences.