Oxypetalum banksii subsp. banksii: a taxon of Asclepiadaceae with an extragynoecial compitum

 The floral morphology of seven Oxypetalum species and, in particular, the spatial relationship between the five stigmatic chambers and two separate ovaries of their flowers with respect to transmission of the pollen tube are studied. In all species, except O. banksii subsp. banksii, floral morphology is similar to that in other Asclepiadeae, and the flowers pollinated with one pollinium develop only one follicle, which means compitum absence. In O. banksii subsp. banksii flowers, the secretory interstaminal tissue lines the inner walls of the stigmatic chambers as in the other species studied, but it also reaches the upper part of the inner surface of the filament tube, where it surrounds the styles, an unprecedented feature for Asclepiadaceae. This tissue secretes nectar and mucilage; the latter acts as transmitting medium for the growth of pollen tubes from pollinia inserted and hydrated in stigmatic chambers (“hyperstigmas”). Mucilage also functions as an extragynoecial compitum: in flowers pollinated with one pollinium both carpels develop into a follicle. Received August 28, 2001; accepted April 9, 2002 Published online: October 14, 2002 Addresses of the authors: Milene Faria Vieira (e-mail: mfvieira@mail.ufv.br), Departamento de Biologia Vegetal, Universidade Federal de Vi?osa, 36571-000, Vi?osa, Minas Gerais, Brazil. George John Shepherd, Departamento de Botanica, Instituto de Biologia, Universidade Estadual de Campinas, C.P. 6109, 13083-970, Campinas, S?o Paulo, Brazil.     

Fruit thinning plums and apricots

The need for crop load reduction to improve fruit size andconsistency of cropping on plum and apricot trees is discussed. Threealternative strategies of achieving this objective are described: byinhibition of flowering, prevention of fruit set or stimulation ofincreased fruitlet abscission. Methods of achieving each of thesestrategies, by manual, mechanical or chemical means are discussed andevaluated.     

Vegetation fluctuation in mediterranean dune ponds in relation to rainfall variation and water extraction

Abstract. The structure of dune ponds hygrophytic vegetation has a spatial organisation in belts around the pond basin, closely related to water level and flooding regime. Doñana National Park has an important representation of temporal dune ponds, which are subjected to rainfall fluctuations and may be suffering the impact of water extraction from the neighbouring tourist resort. Permanent transects in a vegetation complex of five dune ponds have been monitored during a eight year period (1990-1997). This period was characterised by a number of dry years (annual rainfall around 300 mm), located between two wet cycles (800-900 mm). Transects were analysed in 1990 (wet period), 1994 (dry) and 1997 (wet) by hierarchical agglomera-tive clustering. During the dry period hygrophytic species showed regression, with a high mortality of some species like Ulex minor, while the xerophytic species advanced to lower areas. Seedlings of some xerophytic species colonised the dry surface of the pond basin. The lowering of the water table varied in the different ponds, ranging from 312 to 190 cm depending on topography and the distance to the pumping area. The new period of flooding during 1995-96 and 1996-97 cycles provided the opportunity for hygrophytic spe cies to re-establish themselves in their original places. This study suggest that changes in vegetation are caused by the interaction between weather conditions and human disturbance (water extractions). In our example man-made disturbance is more marked during the dry periods while wet periods tend to obscure the effects of water extractions. From a management perspective, long-term monitoring of water table and vegetation structure is revealed as a key procedure to the management of land-water ecotones on pressured areas and threatened habitats.     

Allogibberic acid: An inhibitor of flowering in Lemna perpusilla

Allogibberic acid (I) has been identified as the compound responsible for the inhibition of flowering, increase in frond multiplication rate and decrease in frond size produced in Lemna perpusilla 6746 by autoclaved, unbuffered aqueous solutions of gibberellic acid (VII). 13-Deoxyallogibberic acid (IV), a product of autoclaving aq. GA₇ (VIII) solutions, also inhibits flowering in L. perpusilla and is about 10 times more active than allogibberic acid.     

Rescue of blocked cells by reinnervation in denervated forelimb stumps of larval Ambystoma

Cells of amputated, denervated larval Ambystoma forelimbs dedifferentiate and enter the cell cycle but do not subsequently proliferate sufficiently to form a blastema. The denervated limb stump resorbs slowly until reinnervation stimulates regeneration. We used this system to investigate the fate of cells in denervated limbs which undergo early but limited cycling in response to amputation. In Experiment 1, cells were labeled with [3H]thymidine (3H-T) on Day 4 postamputation (PA)/Day 3 postdenervation (PD). Labeled cells were still present on Day 7 PA, but were less frequently observed on Day 13 PA when the limbs were reinnervated and beginning to regenerate. In Experiment 2 we denervated 1 day preamputation to obtain earlier reinnervation and prevent loss of Day 4 PA labeled cells. Cells labeled with 3H-T on Day 4 PA/Day 5 PD were present throughout the denervation period and most were still present on Day 13 PA. Little or no mitotic activity was found among the labeled cells after the initial round of cycling. The apparent cell cycle block was released upon reinnervation on Days 12 and 13 PA when cycling resumed. Labeled mitotic figures were present on Day 13 PA, and the mitotic index of the labeled population increased as a result of reinnervation. These results demonstrate that blocked cells are rescued by nerves, re-enter the cell cycle, and thus contribute to the reinnervation blastema.