Leaf anatomy of the South African Danthonieae ( Poaceae ) . I . The genus

The anatomical structure, o f the leaf blade as seen in transverse section, and the abaxial epidermis, o f Dregeochloa pumila and D. calviniensis is described and illustrated. A generic description is included and the relationships of the genus are briefly discussed.


INTRODUCTION
U rochlaena Nees is a monospecific genus containing the single species U. pusilla Nees, a small annual char acterized by a dense spike-like panicle embraced by the inflated uppermost leaf sheath.At maturity the culm disarticulates at the uppermost node, complete with the inflorescence and modified upper sheath, and this whole structure acts as a dispersal unit (Chippindall 1955;Clay ton & Renvoize 1986).
U rochlaena is endemic to the Western Mountain Ka roo and the Succulent Karoo of the Vanrhynsdorp, Nieuwoudtville and Calvinia Districts of the Cape Province of South Africa.This restricted distribution range is typi fied by poor soils, low winter rainfall (150 mm or less per annum ), and a low karroid dwarf shrub vegetation with very few perennial grasses (Acocks 1975).
The classification o f Urochlaena has been somewhat inconsistent during the last 30 years.It was initially placed in the Eragrosteae (Chippindall 1955) but its relationships are now considered to lie with the Arundinoideae.Loxton (1976) and Watson et al. (1986) include U rochlaena in the Danthonieae and Clayton & Renvoize (1986) place it in the Arundineae in which tribe the Dan thonieae are included.
U rochlaena pu silla appears to be most closely related to Tribolium utriculosum (Nees) Renv.(= L asiochloa utriculosa Nees) (Chippindall 1955;Clayton & Renvoize 1986).Both are small annuals from the drier, north western parts o f the winter rainfall region, with T. utri culosum extending further northwards into Namaqualand.T. utriculosum always has hairy leaf blades but those of Urochlaena pu silla are either hairy or glabrous except at the bearded sheath mouth (Chippindall 1955).
Little information is available on the leaf anatomy of Urochlaena.De Wet (1960) notes that the anatomy is festucoid but that the epidermis is panicoid.Watson et al. (1986) note that the anatomy is C, without arm and fusoid cells.The microhairs are of the panicoid type, being linear in shape, and the silica bodies are also of the panicoid type, being dumbbell-shaped.It is the purpose of this paper to describe and illustrate the leaf blade anatomy of Urochlaena and to compare its structure with that of the other South African danthonioid grass spe cies.

MATERIALS AND METHODS
Plants of Urochlaena pusilla were collected in the field in the Nieuwoudtville and Vanrhynsdorp Districts.Herbarium voucher specimens were prepared for verifi cation by the National Herbarium (PRE) where they are housed.Leaf segments were immediately fixed in FAA (Johansen 1940).
Transverse sections, 10 /xm thick, were prepared after desilicification in 30 % hydrofluoric acid (Breakwell 1914), dehydration following the method of Feder & O 'Brien (1968) and embedding in Tissue Prep (Fischer Scientific).The sections were stained in safranin and fast green (Johansen 1940).The manual scraping method of Metcalfe (1960) was used to prepare scrapes of the ab axial epidermis.The anatomical structure was recorded photographically using a Reicherdt Univar microscope and Ilford Pan F film.
In the anatomical descriptions which follow, the stan dardized terminology of Ellis (1976Ellis ( , 1979) ) will be used, together with the following abbreviations; Vascular bundle arrangem ent: 3 l 'vbs in leaf section; 2 3 'vbs between consecutive 1 'vbs; no 2 'vbs.All vbs lo cated in the centre of the blade (Figures 3 & 4).Vascular bundle description : 1 ' and 3 'vbs rounded although 3'vbs may tend to be angular in outline (Figure 4); phloem adjoins ibs; metaxylem vessels with very narrow lumens, inconspicuous and less than half the diameter of the obs cells (Figure 4).Vascular bundle sheaths: obs round, complete, with no extensions; cells very small, the lar gest being abaxially located (Figure 4); irregular in size, with thin walls and contain small, unspecialized chloroplasts.lbs entire, without thickened secondary walls.Sclerenchyma: small adaxial strands associated with all vbs (Figure 3); those of the 1 'vbs larger than those of the 3'vbs which consist of only 2 or 3 fibres; abaxial girders associated with the 1 'vbs only; trapezoidal in shape (Fig ure 4).Minute sclerenchyma cap in leaf margin (Figure 3).Fibres not lignified.M esophyU : chlorenchyma nonradiate with no pattern of arrangement (Figure 4); diffu sely arranged with many air spaces; the chlorenchyma cells parenchymatous, rather large but irregular in size and shape; lateral cell count greater than four.No colour less cells present.A daxial epiderm al c e lls: groups of small bulliform cells located at the bases of the furrows; occupy less than / of the leaf thickness; epidermal cells thin-walled; few prickles associated with the adaxial ribs (Figure 2).A baxial epiderm al ce lls: thin-walled, slightly inflated with a thin cuticle; no epidermal appendages visible.

DISCUSSION AND CONCLUSIONS
The leaf anatom y, as described here, agrees with the anatomical details given by De Wet (1960).The transectional anatomy of U rochlaena is typically 'festucoid' whereas the abaxial epidermis is panicoid in several re spects.The outer bundle sheath consists of a single layer o f small, inconspicuous parenchyma cells which do not contain specialized chloroplasts.The chlorenchyma is uniformly and diffusely distributed throughout the mesophyll between the bundles, with no definite pattern of arrangement and with a lateral cell count greater than four.This structure is typical of the non-Kranz anatomy of the pooid grasses and U rochlaena undoubtedly is C3 as reported by Watson et al. (1985).The abaxial epider mis, on the other hand, differs significantly from the pooid type.Microhairs are present, the silica bodies are dumbbell-shaped and not nodular, the long cells have sinuous and not straight walls and the stomata are dome shaped and not parallel-sided.No pooid grass is known to possess microhairs (Watson et al. 1985) and those of U rochlaena are of the panicoid type, being elongated finger-like.The leaf anatomy, therefore, indicates arundinoid affinities and is in full agreement with the classifi cation of the genus in the Arundineae (Clayton & Ren voize 1986).
The anatomy of U rochlaena, with a uniformly distri buted and diffuse chlorenchyma of typical parenchyma cells with large intercellular air spaces and an epidermis with stomata and microhairs, resembles that of some other danthonioid genera from South Africa.Examples are Tribolium Desv., Chaetobrom us Nees, Schismus Beauv., K arroochloa Conert & Tiirpe and some species of Pentaschistis Stapf.The anatomy of these taxa differs significantly from other Cape danthonioid genera such as M erxm uellera Conert, Pentam eris Beauv., Pseudopentam eris Conert and other Pentaschistis species.All these taxa have acicular leaves in which the chlorenchyma consists of small isodiametric cells which are compactly arranged with very small air spaces.The abaxial epider mis also usually lacks stomata and microhairs, and zonation is not evident.This latter type of anatomy has been described in most of the previous papers of this series (Ellis 1980a(Ellis , 1980b(Ellis , 1983(Ellis , 1985a(Ellis , 1985b(Ellis , 1985c(Ellis , 1986)).These two different anatomical types appear to be as sociated with differing ecological conditions.Dantho nioid grasses with acicular leaves and compact mesophyll are all mountain fynbos species growing in oligotrophic soils derived from Table Mountain Sandstone.The unique vegetation of this veld type is characterized by sclerophyllous leaves, and this anatomical type may reflect an equivalent response by these grass taxa to these particular environmental conditions.U rochlaena and the other danthonioid grasses with diffuse mesophyll, on the other hand, favour more fertile soils, such as those of the lowland fynbos and Renosterveld, those derived from granite in the Namaqualand region and several of the Karoo veld types.The M editerranean pooid exotics, which have a very similar transectional anatomy, have also colonized this latter type of environment.Among danthonioids it is only in taxa with the latter type of anatomy that annuals occur, U rochlaena being an exam ple.
Although the leaf anatomy of these two ecological groupings of danthonioid grasses is distinct, it is difficult to ascribe phylogenetic significance to the differences.Pentaschistis is the only genus which includes both ana tomical types and which has species occurring in both these environments.However, the taxonomy of P enta schistis is very poorly understood and it would be unwise to draw phylogenetic conclusions from these observa tions.These two different environments, however, may have exserted diverging evolutionary pressures on these two groups of danthonioid grasses; the relationships of U rochlaena may therefore reasonably be sought among danthonoid grasses with diffuse mesophyll.Indeed, the leaf anatomy of U rochlaena pusilla re sembles that of Tribolium utriculosum and T. echinatum (Thunb.)Renv.very closely indeed.Both always have prominent cushion-based macrohairs which are often also present on U. pusilla.In all other respects the leaf anatomy of these taxa is virtually identical and their affi nities appear to lie with each other.This conclusion based solely on anatomy, is corrobo rated by morphological indications.Chippindall (1955) and Clayton & Renvoize (1986) suggest that U. pusilla and T. utriculosum are closely related because both have tubercle-based hairs as well as capitate hairs on the glumes and lemmas.In T. echinatum the hairs of the glumes are slender and tapering.T. utriculosum has the inflorescence partly enclosed in the uppermost leaf sheath, a condition developed further in U. pusilla.

Silica bodies: short
dumbbell-shaped bodies with wide central portions and rounded ends predom i nate (Figures 6 & 8); somewhat irregular in shape; con fined to costal zones; granules present in silica bodies.