Anomalous vascular bundle sheath structure in Alloteropsis semialata leaf blades

Alloteropsis semialata (R.Br.) Hitchc. o f the Paniceae, would be expected to exhibit typical eupanicoid leaf anatomy with a single bundle sheath o f large parenchymatous cells with specialized chloroplasts and radially-arranged chlorenchyma. Specimens from South Africa showed the following bundle-sheath and mesophyll deviations from the generalized panicoid model: 1. bundle sheath is double; 2. inner bundle sheath is composed o f large cells with specialized chloroplasts; 3. outer sheath consists o f many smaller cells with few, or without chloroplasts; 4. outline o f the thirdorder bundles is tall and narrow; 5. chlorenchyma is not radially arranged. Other specimens o f A. semialata, however, were found to have a large-celled outer bundle sheath devoid o f chloroplasts and with indistinctly radiate chlorenchyma. The genus Alloteropsis has been placed in the tribe Paniceae of the Poaceae. Species of this genus would, therefore, be expected to exhibit a typical panicoid leaf anatom y. Several specimens of A. semialata from South Africa have, however, been observed to exhibit fundamental differences in the structure and com posi­ tion o f the vascular bundle sheaths. The bundlesheath num ber is considered to be a diagnositic character o f im portance in the basic classification of the Poaceae (M etcalfe 1954) and thus the anom alous condition observed in A. semialata w arrants mention. The panicoid grasses are generally characterized by the presence or absence of an inner bundle sheath (Avdulov 1931). The single or outer bundle sheath is composed of relatively few, large, inflated paren­ chym atous cells im parting upon the vascular bundle, sensu Metcalfe (1960), a distinctly angular outline. The chlorenchyma cells o f panicoid grasses are more or less radially arranged. Brown (1961) recognized three panicoid subtypes on leaf anatom y, the Paniceae falling under the eupani­ coid subtype. This group o f tribes is characterized by the inner bundle sheath usually being absent and with the chlorenchyma cells not radially arranged, as seen in transverse section. These cells are normaly tightly packed with few or no air spaces. W ithin the panicoid grasses the main line of evolu­ tion appears to be from the typical eupanicoid struc­ ture towards the development of specialized plastids for starch conversion and storage in the parenchy­ matous sheath cells (Brown 1958). This is accompanied by a loss of the starch-synthesis function by the chloroplasts o f the chlorenchyma, the cells o f which show a corresponding, more regular, radial arrange­ ment. Associated with this development is the com ­ plete loss of the inner bundle sheath. Some species of Panicum, Tricholaena, Oplismenus. Brachiaria and Eriochloa. o f the eupanicoid Paniceae. retain an endodermis-like inner bundle sheath (Brown 1958). Tateoka (1956) found. in some species of Panicum and other genera, that the cells o f the parenchym atous sheath do not have the specialized starch plastids typical o f the Paniceae. Thus there are exceptions to the generalized model for the eupanicoid subtype. Metcalfe (I960) states that single bundle sheaths, radiate chlorenchyma and angular, third-order vas­ cular bundles are diagnostic characters o f the genus Alloteropsis. These characters are repeated in the descriptions given for A. cimicina (L.) S tapf and * Botanical Research Institute. Department o f Agricultural Technical Services, Private Bag X 101, Pretoria. Alloteropsis sp. nov. Specimens o f A. semialata from South Africa showed considerable differences from these generic characters and from the generalized eupanicoid anatomy. The following bundle-sheath and mesophyll differences were observed in specimens of A. semialata (Figs. 1-4, 6, 7): 1. the bundle sheath o f all orders o f vascular bundle consists o f two layers o f parenchym atous cells. 2. the inner bundle sheath is composed o f inflated, parenchym atous cells with a large, centrally-situated chloroplast. These cells are larger than those o f the outer sheath. The inner bundle-sheath cells are unthickened except in first-order vascular bundles where the radial and inner tangential walls are slightly thickened. 3. the outer bundle sheath is comprised o f num erous smaller, thin-walled cells without, or with a few normal chloroplasts. 4. the outline o f the third-order vascular bundles is not angular, but tall and narrow. 5. the chlorenchyma cells are not tabu lar and radially arranged as seen in section, but are hexagonal to inflated in shape, with no definite pattern of arrangem ent. Furtherm ore, these cells are distinctly larger than the outer and inner bundle-sheath cells. This anom alous structure was observed in the following specimens, the voucher specimens being deposited in the National Herbarium. Pretoria. T r a n s v a a l .—2528 (Pretoria): N ational Botanical G ardens, Brummeria (-C A ) 5-11-1969 R. P. Ellis 2\ 1-12-1972 R. P. Ellis 1244; Faerie Glen (-C D ) 22-11-1972 R. P. Ellis 721. N a t a l .— 2729 (Volksrust): C lontarf Siding, Newcastle (-D B ) 12-11-1969 R. P. Ellis 36. This structure bears a superficial resemblance to the bundle sheaths o f the genus Aristida o f the aristidoid subtype of the panicoid grasses (Brown 1958). Aristidoid grasses are characterized by two paren­ chyma sheaths and no endodermis-like inner sheath, but the chlorenchyma is distinctly radiate. In the sheaths of Aristida the larger inner-sheath cells also possess abundant chloroplasts. more than in the smaller, outer bundle-sheath cells (Lommasson 1957). O ther specimens o f A. semialata were found to have a large-celled, outer bundle sheath devoid of chloroplasts. a typical mestome or inner bundle sheath around the larger bundles and the chloren­ chyma indistinctly radiately arranged (Figs. 5, 8). 274 BUNDLE SHEATH STRUCTURE IN ALLO TERO PSIS SEM IALATA LEAF BLADES F i g s . 1-5.— Leaf blade outline o f Alloteropsis semialata in transverse section. A l 1 x 6 ,2 5 , bright field. Fig. 1, Ellis 2. Fig. 2, Ellis 721. Fig. 3, Ellis 1244. Fig. 4, Ellis 36. Fig. 5, Ellis 733. The specimens exhibiting this structure are: T r a n s v a a l .— 2530 (Lydenburg): Belfast (-C A ) 6-2-1904 Burtt-Davy 1323. 2531 (K om atipoort): Ida Doyer N ature Reserve (-C C ) 21-1-1971 R. P. Ellis 378 and N a t a l .— 3030 (Port Shepstone): Eureka Farm , Izotsha (-C D ) 20-12-1971 R. P. Ellis 733. These specimens conform reasonably closely to the Metcalfe (I960) description for the genus except for the lack o f chloroplasts in the sheath cells and the presence o f a double bundle sheath. The occurrence of two distinct types of anatomical structure within a single species is unexpected and further investigation is required. Chippindall (1955) notes that A. semialata is exceedingly variable in inflorescence size and form and it is not im probable tha t these anatom ical differences are correlated with morphological ones. REFERENCES A v d u l o v , N. P., 1931. Karyo-systematische Untersuchung der Familie Gramineen. Bull. Appl. Bot. Genet. Plant Breed., Suppl. 44: 1-428. B r o w n , W. V., 1958. Leaf anatomy in grass systematics. Bot. Gaz. 119: 170-178. B r o w n , W. V., 1961. Grass leaf anatomy: Its use in syste­ matics. Recent Advances in Botany. I: 9th International Bot. Congress 1959: University o f Toronto Press. C h i p p i n d a l l , L. K. A., 1955. In Meredith et al. Grasses and Pastures o f South Africa. Johannesburg: Central News Agency. Lo m m a s s o n , R. C., 1957. Vascular bundle sheaths in the genus Aristida. Phytomorphology 7 (3-4): 364. M e t c a l f e , C. R. 1954. Recent work on the systematic anatomy o f the Monocotyledons. Kew. Bull. 4: 523-532. M e t c a l f e , C. R., I960. Anatomy o f the Monocotyledons. /. Gramineae. Oxford: Clarendon Press. T a t e o k a . T . , 1956. Re-examination o f anatomical charac­ teristics o f the leaves in Eragrostoideac and Panicoidcae (Poaceae). J. Jap. Bot. 31: 210-218.

The genus Alloteropsis has been placed in the tribe Paniceae o f the Poaceae.Species o f this genus w ould, therefore, be expected to exhibit a typical panicoid leaf anatom y.Several specimens o f A. semialata from South Africa have, however, been observed to exhibit fundam ental differences in the structure and com posi tion o f the vascular bundle sheaths.The bundlesheath num ber is considered to be a diagnositic character o f im portance in the basic classification o f the Poaceae (M etcalfe 1954) and thus the anom alous condition observed in A. semialata w arrants m ention.
The panicoid grasses are generally characterized by the presence or absence o f an inner bundle sheath (A vdulov 1931).The single o r outer bundle sheath is com posed o f relatively few, large, inflated paren chym atous cells im parting upon the vascular bundle, sensu Metcalfe (1960), a distinctly angular outline.The chlorenchym a cells o f panicoid grasses are more or less radially arranged.Brown (1961) recognized three panicoid subtypes on leaf anatom y, the Paniceae falling under the eupani coid subtype.This group o f tribes is characterized by the inner bundle sheath usually being absent and with the chlorenchym a cells not radially arranged, as seen in transverse section.These cells are norm aly tightly packed with few or no air spaces.W ithin the panicoid grasses the main line o f evolu tion appears to be from the typical eupanicoid struc ture tow ards the developm ent o f specialized plastids for starch conversion and storage in the parenchy m atous sheath cells (Brown 1958).This is accom panied by a loss o f the starch-synthesis function by the chloroplasts o f the chlorenchym a, the cells o f which show a corresponding, m ore regular, radial arrange ment.Associated with this developm ent is the com plete loss o f the inner bundle sheath.Alloteropsis sp.nov.Specimens o f A. semialata from South A frica showed considerable differences from these generic characters and from the generalized eupanicoid anatom y.
The following bundle-sheath and mesophyll differences were observed in specimens o f A. semialata (Figs.1-4, 6, 7): 1. the bundle sheath o f all orders o f vascular bundle consists o f two layers o f parenchym atous cells.
2. the inner bundle sheath is com posed o f inflated, parenchym atous cells with a large, centrally-situated chloroplast.These cells are larger than those o f the outer sheath.The inner bundle-sheath cells are unthickened except in first-order vascular bundles where the radial and inner tangential walls are slightly thickened.
3. the outer bundle sheath is com prised o f num erous sm aller, thin-walled cells w ithout, or with a few norm al chloroplasts.
4. the outline o f the third-order vascular bundles is not angular, but tall and narrow . 5. the chlorenchym a cells are not tab u lar and radially arranged as seen in section, but are hexagonal to inflated in shape, with no definite pattern o f arrangem ent.Furtherm ore, these cells are distinctly larger than the outer and inner bundle-sheath cells.This anom alous structure was observed in the following specimens, the voucher specimens being deposited in the N ational H erbarium .Pretoria.This structure bears a superficial resem blance to the bundle sheaths o f the genus Aristida o f the aristidoid subtype o f the panicoid grasses (Brown 1958).A ristidoid grasses are characterized by two paren chym a sheaths and no endoderm is-like inner sheath, but the chlorenchym a is distinctly radiate.In the sheaths o f Aristida the larger inner-sheath cells also possess ab u n d an t chloroplasts.m ore than in the smaller, outer bundle-sheath cells (L om m asson 1957).
O ther specimens o f A. semialata were found to have a large-celled, outer bundle sheath devoid of chloroplasts.a typical m estome or inner bundle sheath around the larger bundles and the chloren chyma indistinctly radiately arranged (Figs. 5, 8).The occurrence o f tw o distinct types o f anatom ical structure w ithin a single species is unexpected and fu rther investigation is required.Chippindall (1955) notes th a t A. semialata is exceedingly variable in inflorescence size and form and it is not im probable th a t these anatom ical differences are correlated with m orphological ones.* f Some species o f Panicum, Tricholaena, Oplismenus.Brachiaria and Eriochloa.o f the eupanicoid Paniceae.retain an endoderm is-like inner bundle sheath (Brown 1958).T ateoka (1956) found. in som e species o f Panicum and oth er genera, that the cells o f the parenchym atous sheath do not have the specialized starch plastids typical o f the Paniceae.Thus there are exceptions to the generalized model for the eupanicoid subtype.Metcalfe (I960) states th at single bundle sheaths, radiate chlorenchym a and angular, th ird-order vas cular bundles are diagnostic characters o f the genus Alloteropsis.These characters are repeated in the descriptions given for A. cimicina (L.) S tapf and * Botanical Research Institute.Department o f Agricultural Technical Services, Private Bag X 101, Pretoria.