GROWTH, SURVIVORSHIP, AND POPULATION SIZE IN THE LAND SNAIL RHAGADA CONVICTA COX, 1870 (PULMONATA: CAMAENIDAE) FROM A SEMIARID ENVIRONMENT IN WESTERN AUSTRALIA

Basic characteristics of a population of the camaenid land snailRhagada convicta were studied in a semiarid environment nearNorth West Cape, in Western Australia. Consistent evidence frommark-recapture studies and analysis of size-frequency distributionindicates an average annual increase in shell diameter of 3.6mm, with an average period of 5 years from hatching to reachadult size. These rates are low, compared with most other landsnails studied, including cama-enids from wetter environments.Estimates of mortality rate and the rate of recruitment intothe adult population indicate that the snails live an averageof approximately 5 years after reaching adult size, which meansthat the period of turnover for this population is approximately10 years. The population density was estimated to be 0.8 adultsper m², with a total population size (± s.e.) of 875± 164. The population is very localised, with an estimateddiameter of the neighbourhood area of only 38 m. (Received 28 August 1990; accepted 12 February 1991)     

The Effect of Ultraviolet Radiation on the Germination of Nosema algerae Vávra and Undeen (Microsporida: Nosematidae) Spores1

Spores of Nosema algerae Vávra and Undeen were subjected to various dosages of 254 nm ultraviolet radiation (UV). Very high dosages of UV were required to block germination. Germination was normal immediately after UV dosages of 0.2 to 1.0 J/cm², followed by a delayed effect in which both percentage germination and the intrasporal concentration of trehalose decreased with time after UV exposure. Although a few spores were germinated, most of them were inactivated (rendered temporarily unable to germinate) by exposure to UV of 1.1 J/cm². Ultraviolet radiation between 1.1 and 3.4 J/cm² stimulated spores to germinate. However, spores were completely unable to germinate immediately after exposure to dosages above 3.8 J/cm². Ammonia had little effect on stimulation by UV but was inhibitory to germination after stimulation had occurred. These results demonstrate that UV behaves like a germination stimulus and are discussed in terms of the hypothesis that germination is initiated by the breakdown of barriers between trehalose and trehalase.     

Perianth Abscission in Montbretia (Crocosmia x crocosmiiflora)

The anatomy, histochemistry, kinetics and hormonal regulationof perianth abscission in Crocosmia x crocosmiiflora (Montbretia)has been investigated. The abscission zone is anatomically welldefined, with cell divisions occurring in this region at anthesis.Abscission is first detectable 3 d after perianth opening, whenthe walls of a group of cells beneath the adaxial epidermisshow reduced staining with polyanion-specific stains, and adecline in penanth break strength also occurs. Abscission isachieved by cell wall breakage in thc abscission zone, whichprogresses eccentrically from the adaxial epidermis throughthe abscission zone, rather than the separation of intact cellsas occurs in flowers of dicotyledons. Experiments on detachedflowers suggest similarities in the hormonal regulation of abscissionin Crocosmia to that of dicotyledons, in that an ethylene promotion,and possibly an auxin inhibition, mechanism may exist in Crocosmia.Ovary expansion occurs throughout the development and senescenceof unpollinated flowers, but does not appear to be the solecause of wall breakage in the abscission zone. It is suggestedthat hormonally regulated wall hydrolases weaken the cell wallsin the abscission zone, and allow wall breakage and subsequentabscission to occur. Cdrocosmia x crocosmiiflora, Montbretia, anatomy, breakstrength, cell wall changes, histochemistry, flowers, monocotyledons, perianth, senescence, ethylene, auxin     

Reasons for poor establishment of direct re-seeded grassland

These experiments suggest a number of causes for the poor establishment of direct reseeded grass. An adequate supply of moisture to the seed is of prime importance and burial of trash in drill slits may impair this supply by preventing seed/soil contact directly or by preventing closure of the drill slit. Management techniques which aim to reduce the burden of trash in areas to be sown by direct drilling, usually result in improvements in emergence. This is achieved by improved seed/soil contact, reduced evaporation of water from an open drill slit and lowered probability of phytotoxic leachates being produced. It is possible that benefits might be achieved were it possible to select for sowing seed of proven high vigour, i.e. seed that establishes well in poor conditions.