Life Cycle of Polytomella caeca (Phytomonadida, Polyblepharidae)

SYNOPSIS. Polytomella caeca (Phytomonadida, Polyblepharidae) reproduce by binary fission and sexual reproduction. The sexual process increases in frequency as the population becomes denser. Normally 2 monoecious isogametes engage their anterior flagella prior to fusion, which occurs by merging of the cells from their anterior to their posterior ends. This is followed by a period of zygotic enlargement before cleavage of the zygote into 4 daughter cells. Encystment occurs throughout the flagellates' bloom until they represent 80% of the individuals at the end of the bloom. Cysts are a resistant, not reproductive stage. The observations were made over long periods of time by following individual flagellates in microcultures and by viewing living cells taken at various stages from larger cultures.     

The role of the sheath tube in the development of expanding leaves in perennial ryegrass

A possible morphogenic effect of leaf sheaths on subsequent leaf development was investigated by varying sheath tube (pseudostem) length in plants of perennial ryegrass (Lolium perenne) cv. Talbot by either incising longitudinally or excising the distal portion of the sheath tube, while leaving the basal length of the tube intact. The tube was maintained at predetermined lengths by incising and excising new growth daily. The youngest rapidly expanding leaf was allowed to grow through the tube and was measured when fully expanded. Reducing tube length by excision or by incision from 60 mm to just above the cowl leaf on the apex reduced lamina length by 87% and 77% respectively. Over all tube lengths, laminae in incised treatments were almost twice as long as those in excised treatments. Sheath length followed a similar pattern. The effect on developing leaves of artificially extending sheath tubes (previously excised to 15 mm) to 30 or 45 mm with foil was similar to that of initially excising sheath tubes to 30 or 45 mm. The shorter the sheath tube (reduced by incision) through which the leaves had to grow, the shorter the cells, especially in the laminae. The estimated cell number per row along the length of the laminae ranged from 190 in tillers (shoots) with a very short tube (just above the cowl leaf) to 454 in intact control tillers. It is concluded that the sheath tube has a morphogenic influence on the development of subsequent leaves due to the change in environment of the leaf lamina on appearance, affecting both cell elongation and cell division.     

THE HEAT RESISTANCE OF ESCHERICHIA COLI CELLS FROM CULTURES OF DIFFERENT AGES

SUMMARY: From the mortality curves of Escherichia coli cells heated at 55° in Ringer's solution both Decimal Reduction Times (DRTs) and 99–9% mortality times were obtained. In terms of these measures of heat resistance, cells harvested from broth cultures 0–8 hr old were more susceptible than those from more mature cultures. The time of commencement and the approximate duration of the logarithmic phase of growth of the organism in broth were determined from growth curves, and it was observed that the heat resistance was minimal during that phase.
Death rates were not always uniform for the whole of a given population. Particularly among young cultures, a period of rapid death was frequently followed by the slower death of a relatively small number of survivors. In one instance only was an initial period of slow death followed by one of more rapid death.     

Effect of R-plasmid RP1 and Nutrient Depletion on the Resistance of Escherichia coli to Cetrimide, Chlorhexidine and Phenol

The minimum inhibitory concentation of cetrimide and chlorhexidine for Escherichia coli is altered when R-plasmid RPI is inserted into it. Using disinfectant-agar plates as a test system, the sensitivity of Esch. coli to these disinfectants and to phenol was found to vary, not only with the presence of RPI, but also with the nutritional status of the inoculum. The different inocula also varied in their ability to bind H⁺ and there was some correlation between this property and chlorhexidine sensitivity.