Leaf anatomy of the South African Danthonieae (Poaceae). Merxmuellera stricta

The anatomical structure, of the leaf blade as seen in transverse section, and of the abaxial epidermis, of Merxmuellera stricta (Schrad.) Conert is described and illustrated. In this variable species four distinct ana­ tomical “forms” are recognized viz. the typical M. stricta form, the Cathedral Peak form, the Drakensberg form and the alpine form. The alpine and Cathedral Peak forms have recently been described as M. guillarmodiae Conert (1975). The degree of anatomical differentiation of these “forms” resembles the situation described in M. disticha (Nees) Conert (Ellis, 1980). Populations of both M. stricta and M. disticha from the Drakensberg mountains display extensive anatomical diversification which appears to be correlated with environmental factors. In addition, morphological differences are exhibited as well and the anatomical “forms” of M. stricta probably warrant taxonomic recognition. large diversification anatomique correlation facteurs milieu. differences morphologiques

. M . stricta is also com m on in the fynbos com m unities o f the so u th w estern Cape.
M . stricta is a variable perennial, form ing coarse, wiry tufts. C hippindall (1955) states th a t " T here is considerable variation in th e p lants referred to D. stricta, and it is possible th a t they com prise m ore th an one variety" . In the north-w est M . stricta may be confused with M . dura (S tap f) C o n ert, b u t M . stricta can be recognized by the g labrous cond itio n o f the lem m a at the point o f insertion o f the central awn and is distinct anatom ically (Ellis, in prep.). In the north-east, in the D rakensberg m ountains, a situation exists, sim ilar to th a t observed in M . disticha (Ellis, 1980), with three additional an ato m ical " form s" being present.
These anatom ical " form s" a p p ear to be correlated w ith m orphological characters and h a b ita t differences. as C onert (1975) has independently described a new species, M . guillarmodiae C onert, from M . stricta collections from the alpine region o f the D rakensberg.
U nfortunately, the specimens cited by C onert (1975) as belonging to M . guillarmodiae fall into tw o o f the anatom ical categories recognized in the present study, while the rem aining tw o " form s" fall in M . stricta as currently constituted. If the precedent created by the description o f M . guillarmodiae is to be followed, it implies th a t a fu rth er tw o species require description. Similarly, by the sam e token, tw o new species, present ly referred to M . disticha, also w arrant description (Ellis, 1980).
In the present context M . stricta is viewed in its widest sense, and for convenience M . stricta sens. lat. has merely been sub-divided into four " form s" for des criptive purposes: the typical form ( M . stricta), the C athedral Peak form ( M . guillarmodiae), the D rakens berg form ( M . stricta) and the alpine form (M . guillarmodiae). Each o f these " form s" exhibits charac teristic leaf anatom y and epiderm al structure which will be described and discussed according to the term inology o f Ellis (1976;1979).
In the anatom ical descriptions which follow, the following abbreviations will be used: vb/s-vascular bundle/s l'vb/s-first ord er vascular bundle/s 2'vb/s-second o rder vascular bundle/s 3'vb/s-third o rder vascular bundle/s ibs-inner bundle sheath; m estom e sheath obs-other bundle sheath; parenchym a sheath. occurs in D rak en sb erg form ( Fig. 9) and to 45° in the alpine form (Fig. 12). L am ina sym m etrical a b o u t the m edian vb. 9-13 vbs present in leaf section (T able 1) with th e alpine form always with 9 vbs (Figs 10-12) and ty pical M . stricta w ith 11 o r 13 (Figs 1-3). A daxial ch an n el an extrem ely narrow and deep cleft in the iyp ical and C athedral Peak form s and is n a rro w e r th an the lam ina thickness in the D rak ens berg and alp in e form s when infolded. Leaves narrow (< 1 ,1 m m wide) when folded. Ribs and furro w s: adaxial furrow s o f variable depth b u t co n stan t for each o f th e " form s" (Table 1) (Fig. 5). Lysigenous cavities present. M etaxylem vessels thickened, circular and very narrow . Vascular bundle sheaths: obs circular or elliptical b u t n o rm ally horse shoe shaped due to wide abaxial in terru p tio n s. A daxial extensions present but cell com position differs in each o f the " form s" . Obs cell shape variable from round to elliptical but all cells in a given leaf sim ilar in shape. Obs cells conspicuous but n o t larger th a n the All with round silica bodies.

DISCUSSION A N D CONCLUSIONS
All th e specim ens exam ined in this study have the lateral first o rd er vascular bundles located adjacent to one a n o th e r and lack the single th ird ord er bundle in tersp aced betw een them w hich is characteristic o f M . disticha (Ellis, 1980). M . dura, considered to be closely allied to M . stricta (C hippindall, 1955;De W et, 1960), does n o t share this ch aracteristic and has, th erefo re, been excluded from the present considera tions.
A n ato m ic ally the leaf stru ctu re o f th e typical M . stricta fo rm is rem arkably c o n sta n t th ro u g h o u t its wide d istrib u tio n a l area (Fig. 33). T he only an ato m i cal difference w orthy o f m ention is the developm ent o f a c o n tin u o u s abaxial hypoderm al sclerenchym a layer (Fig. 1) w ith the resu ltan t exclusion o f th e intercostal zones a n d sto m ata (Fig. 13). This structure was p resent in 40% o f the specim ens exam ined in this study, a n d occurs in p lants from widely scattered localities, th ro u g h o u t the range o f this form , and does n ot a p p e a r to be correlated w ith any obvious environ m ental fac to r. M orphologically there is a definite tendency fo r these specim ens to have shorter glum es [(11-)  stantly linked and, on the basis o f glum e length, these shorter spikelets tend to resem ble the C athedral Peak form o f M . guillarmodiae (Table 1). However, the M . stricta form has distinctly longer aw nsaveraging 11,1 mm as against 9,8 mm for the C athedral Peak form . This form , how ever, does n ot have a continuous hypoderm al sclerenchym a layer (Fig. 4) and has num erous stom atal files (Figs 16-18).
The anatom y o f the C athedral Peak form is unm istakeable, due to the shape o f the adaxial groove which lacks ribs and furrow s, (Figs 4-6) and due to the occurrence o f dum b-bell shaped silica bodies (Figs 16-18). This type o f anatom ical structure appears to be strongly correlated All m orphological indications are, therefore, th a t this C athedral Peak form is closely linked to the typical M . guillarmodiae or alpine form . However, one specimen, Trauseld 833, does n o t exhibit any o f these M . guillarmodiae spikelet characteristics. A lthough the adaxial groove and silica bodies con form , it differs anatom ically from the rem ainder o f the C athedral Peak form sam ple in th a t it lacks adaxial bundle sheath extensions and thus the m eso phyll d istrib u tio n is continuous adaxially and n o t restricted to separate tall and narrow groups (Fig. 6). This specim en was collected a t G iants Castle and, although this area was revisited, no plants w ith C athedral Peak form anatom y or m orphology were located. T he D rakensberg form , w hich Trauseld 833 resembles m orphologically, is com m on in this area, but all specim ens collected had the distinctive D rakens berg form anatom y. Specimens, conform ing an atom i cally and m orphologically w ith the C athedral Peak form , do occur a t H ighm oor to the south o f G iants C astle, e.g. Du Toit 2500. F u rth er collections from the southern D rakensberg are required to help clarify the relationships o f this C athedral Peak form . It has been grouped under M . guillarmodiae (C onert, 1975) together with the alpine form , which it closely resembles m orphologically. A natom ically, how ever, it has m ost in com m on with the D rakensberg form . Both these latter " form s" have sim ilar sclerenchym a girders and bundle sheath extensions but, m ost significantly, b oth have d um b bell shaped silica bodies.
The D rakensberg form , on the other hand, although resem bling the C athedral Peak form anatom ically, differs significantly m orphologically. The back o f the lem m as are glabrous along the centre, as in typical M . stricta, and the lower glumes are m uch longer (Table 1) -30, Pentaschistis tysonii. (29, Ellis 3302, x250, third order bundles absent; 30, Ellis 3319, x400,  bundle sheath cells distinct.) 31-32, Pentaschistis sp. (31, Ellis 1409, xlOO, no third order bundles between lateral  first order bundles; 32, Ellis 3292, x 400.) This anatom ical difference, although distinct and consistent, may, how ever, n o t be as significant as it initially appears. It can be sim ply explained by opposite developm ental trends in respect o f only a single ch aracter-a single adaxial furrow on either side o f the m edian vascular bundle. The epiderm is also closely resem bles th a t o f the C athedral Peak form as b oth have dum b-bell shaped silica bodies (Figs 16-21).
T he anatom ical indications are, therefore, th a t the D rakensberg form resem bles the C ath ed ral Peak form m ore closely th an either o f the oth er two form s. T hese sim ilarities are n o t co rro b o rated by spikelet structure, how ever, and in this respect the D rak en s berg form displays typical M . stricta structure.
Some D rakensberg form specim ens from the eastern C ape {Ellis 243; S to ry 4522), how ever, anatom ically dem o n strate this relationship w ith typical M . stricta. T hese specim ens have ro u n d silica bodies, w ith characteristic M . stricta type epiderm al structure but in section the an ato m y is typical o f the D rakensberg form . This observation, therefore, provides an im p o rt a n t clue as to the affinities o f this form and provides an anatom ical link to su p p o rt the m orphological evidence. A degree o f m orphological and anatom ical g ra d a tio n appears to occur betw een these two form s in the areas o f sym patry in the n orth-eastern C ape and once again fu rth er collecting is required to help confirm affinities between the D rakensberg, C athedral Peak and typical M . stricta form s.
The anatom y o f the alpine form specim ens com pares very favourably w ith th a t o f th e type specimen o f M . guillarmodiae (Jacot-G uillarm od 3734). This form appears to be distinct, b o th anatom ically and m o rp h o logically, from M . stricta (C onert, 1975) although the C athedral Peak form is interm ediate in m ost spikelet characters (Table 1). In ad d itio n it has specialized h a b ita t requirem ents, being restricted to the higher alpine zone o f the D rakensberg, often associated with boggy conditions. It is, therefore, n o t spatially associated w ith any o f th e o th er form s.
T his alpine form o f M . guillarmodiae exhibits striking resem blances, in h a b ita t preferences, grow th form and anatom y, w ith the alpine bog form described in M . disticha (Ellis, 1980). A natom ical sim ilarities are rib and furrow d istrib u tio n and form , m esophyll configuration and epiderm al structure. The only difference is in the p attern o f arran g em en t o f the v arious orders o f vascular bundle along the w idth o f the lam ina. T his arran g em en t differs in the two " form s" b u t corresponds w ith the p attern s found in either typical M . stricta o r typical M . disticha. This difference is correlated with differences in inflore scence characters-a co ntracted panicle in M . stricta and a distichous spike in M . disticha-and appears to indicate the relationships o f the alpine " form s" . This seems to be an excellent exam ple o f convergent evolution in response to sim ilar environm ental conditions.
The m ost obvious m orphological differences betw een the specimens o f the fo u r " form s" o f M . stricta, recognized and exam ined in this study, are briefly sum m arized in T able 1. Typical M . stricta and the D rakensberg form have significantly longer low er glumes th an do the alpine and C athedral Peak form s. T he longer glumes are associated w ith longer awns on the low er lem m a except in the C athedral Peak form which is interm ediate betw een the alpine form and the o th er tw o. The C athedral Peak and typical M . stricta form s are glabrous on the back o f the lower lem m a along the central vein up to the base o f the central awn, although, occasionally, a few scattered hairs m ay be present in M . stricta type specimens. In both the alpine and C athedral Peak form s the back o f the lem m a is hairy-sparsely hairy with hairs up to 2 mm long in typical M . guillar modiae (the alpine form ) b ut densely hairy with longer hairs (2,0-3,2 mm long) in the C athedral Peak form .
M orphological characters, therefore, indicate two groups in M . stricta sens. lat.-the typical M . stricta and D rakensberg form s sharing certain characters whereas, the same characters differ considerably in the alpine and C athedral Peak form s. This grouping is not confirm ed by anatom ical evidence which indicates close relationships between the C athedral Peak and D rakensberg form s w ith typical M . stricta and the alpine form s being distinct. How ever, a few specimens with anom alous anatom y tend to break dow n the rigid anatom ical divisions and, thereby, add substance to the m orphological grouping.
The acceptance o f the m orphological groupings as reflecting affinities, implies th a t silica body structure is o f no significance in this instance. This would norm ally be considered unlikely as silica bodies are usually of considerable value taxonom ically (M etcalfe, 1960). Typical M . stricta specimens have classic rounded or elliptical festucoid-type silica bodies whereas, both the D rakensberg and C athedral Peak form s have panicoid-type, dum b-bell shaped bodies (Clifford & W atson, 1977). All other indications are th at typical M . stricta and the D rakensberg form are closely related except for silica body shape. In fact, these two " form s" o f the sam e species possess silica bodies supposedly characteristic o f different tribal groupings (P rat, 1932; 1936). De W et (1954De W et ( , 1956 notes the mixed character o f the epiderm is in M . stricta b u t actually refers to the association of m icro-hairs (a panicoid character) w ith the festucoid elliptical silica bodies. Two panicoid epiderm al characters m ay, therefore, occur in M . stricta sens, lat. b ut this is n o t supported by other anatom ical evidence.
The recent description o f M . guillarmodiae (C onert, 1975) as a separate species initially appears justified on the anatom ical evidence presented in this paper. However, certain M . guillarmodiae specimens, included here in the C athedral Peak form , differ dram atically from the type o f M . guillarmodiae (the alpine form ) and, applying the same criteria appear to m erit specific status in their own right. The D rakens berg form w ould then also w arrant specific status. This implies th a t a further tw o species require descrip tion, as is the case in M . disticha where an alm ost identical anatom ical situation exists (Ellis, 1980). However, especially in M . stricta, these anatom In addition, tw o o f the four setaceous-leaved Pentaschistis species from this same area, display rem arkable anatom ical sim ilarities with M . stricta sens. lat. (Figs 25-32). P. basutorum S tap f (Fig. 25) and P. fibrosa S tapf (Figs 27 & 28) have typical Pentaschistis anatom y with thin-w alled o u ter bundle sheath cells and som ew hat diffuse m esophyll. The bundle sheath extensions and sclerenchym a associated with the th ird order bundles differ considerably from M . stricta sens. lat. (Fig. 26). P. tysonii S tap f (Figs 29 & 30) and Pentaschistis sp (Figs 31 & 32), on the other h and, display typical M erxm uellera type anatom y, and, on anatom ical grounds only, ap p ear to have m ore in com m on with M erxm uellera and espe cially the M . stricta group th an they do w ith Pentas chistis.
The follow ing Pentaschistis specimens were ex am in ed : P. basutorum F o r the above reasons the description o f num erous new M erxm uellera species from this sum m er rainfall area is to be cautioned against for the tim e being. The indications are th at this tem perate region o f high altitudes has only relatively recently been colonized by these typically w inter rainfall grasses from the south. A daptive radiation appears to be actively continuing and the taxonom ic picture is n o t at all clear. F u rth e r studies, especially those o f a biosystem atic and autecological nature, are needed, w ithin M erxm uellera and closely related genera in the D anthonieae, before reliable taxonom ic conclusions can be reached. A t present, the assigning o f specific rank to any o f these anatom ical " fo rm s" can n o t be fully justified. However, for practical purposes, each o f these anatom ical " form s" , described in b o th M . stricta and M . disticha (Ellis, 1980), deserve taxonom ic recognition but infraspecific groupings are recom m ended until the status o f this genus in this area is better understood.
In arriving at a final conclusion, it m ust be rem em bered, th a t, in both M . stricta and M . disticha, the anatom ical differences between the " form s" are o f considerable m agnitude, disjunct and are correlated with oth er anatom ical, m orphological and ecological characteristics. In m any instances these differences are, therefore, greater than are those between o th er M erxm uellera species and even between some o f the genera o f the D anthonieae.