Panicum simulans ( Paniceae , Poaceae ) , a new species from southern Africa and its leaf anatomy

Panicum simulans Smook from northern Namibia is formally described. The morphology and leaf blade anatomy in transection and surface view, as observed under the light microscope, are described. The species is compared to others with which it has been confused in the past. P. simulans can be readily distinguished from P. novemnerve Stapf and P schinzii Hack, on the basis of both anatomical and morphological characters such as the venation of the lower glume and the photosynthetic anatomy, which is of the NAD-me subtype of the C4 photosynthetic pathway. On this basis it appears that P simulans is closely allied to the P. coloratum L. group of species and should be placed in section Panicum of subgenus

three species clearly and to justify the recognition of P simulans (Table 1).The specific epithet refers to the superficial similarity between P simulans, P schinzii and P. novemnerve.

LEAF ANATOMY OF PANICUM SIMULANS
In order to obtain further evidence relating to the taxo nomic status and position of P. simulans, a detailed anatomical study was undertaken.The methods used are described in Ellis (1988) and the terminology used is defined in Ellis (1976Ellis ( & 1979)).The following abbrevia tions will be used:

Abaxial epidermis in surface view
Zonation: evident but costal zones overlying 3'vbs often without silica bodies as a result of absence of sclerenchyma strands in association with these bundles.Intercostal long cells: elongated (length more than 3 X longer than width), side walls parallel to outwardly bowed, end walls vertical; anticlinal walls very thin and slightly undulating, with degree of sinuosity varying with plane of focus; cell shape varies across individual intercostal zones with two files of bulliform-like cells per intercostal zone; these bulliformlike cells actually represent central files of two intercostal zones but costal zones are poorly differentiated (Figure 4C, D All specimens from Namibia previously assigned to P schinzii have now been re-identified as P sim ulans.There is one exception, Volk 2672.However, there is no habitat information for this specimen, and it may have been a weed from a cultivated land and imported with the crop.
Panicum schinzii and P. simulans both tend to be hydrophytic, but they are clearly separated geographically.Their natural populations were undoubtedly originally allopatric, but it can be expected that, with increasing agriculture under irrigation, P. schinzii will become established in the range of P. simulans.

Figure 2
Figure 2 shows the known distribution of P simulans.It is restricted to northern Namibia but may possibly also occur in southern Angola.It is associated with various vegetation types ranging from mopane woodland to mopane mixed with Acacia, Combretum or Terminalia to sweet grassland on limestone soils such as occur around Etosha Pan.
FIGURE 3.-Leaf blade anatomy of Panicum simulans as seen in transverse section.A, outline of specimen without keel; median vascular bundle only (arrowed), this bundle structurally indistinguishable from other first-order vascular bundles; B, outline of specimen with distinct keel incorporating four vascular bundles and adaxial colourless parenchyma tissue; C, typical C4 photosynthetic anatomy with prominent paren chyma sheaths; D, specimen showing distinct centripetal location of Kranz chloroplasts in outer bundle sheath cells; E, dense chloroplasts in outer bundle sheath cells and radiate chlorenchyma, note adaxial papillae; F, typical NAD-me type photosynthetic anatomy.A, B, x 100; C, D, x 250; E, F, x 400.A, C, Smook 5108' , B, Smook 5120; D, Gibbs Russell A Smook 5268; E, Smook 5153; F, Smook 5110.
). Very few tall and narrow short cells present.Stomata: 2 or 4 files of low dome-shaped stomata per intercostal zone; usually separated by a single elongated interstomatal cell.Papillae: absent.Prickles: absent.Microhairs: bicellular with dehiscent distal cell; only basal cells remain, basal cell elongated with length much greater than width; occur in central files of intercostal zones (Figure 4D).Macrohairs: absent.Silica bodies: very irregular dumbbell-shaped; horizontally elongated; same width as adjacent long cells.Costal short cells: 1 or 3 files of cells comprise costal zones; central file with silica bod ies, these alternate irregularly with short to long short cells.Costal zones often not associated with 3'vbs, only present over l'vbs.DISCUSSION In general appearance P. simulans closely resembles P schinzii, particularly those forms o f both taxa that have rounded or cordate leaf bases.However, the hairy nodes (Figure IB) and pointed apex of the spikelets (Figure ID, E) and colour of plant (yellowish green vs. green) of P. simulans clearly separate these two taxa (Table 1) (Gibbs Russell et al. 1990).