303 research outputs found
Intrusive growth of sclerenchyma fibers
No full textIntrusive growth is a type of cell elongation when the rate of its longitudinal growth is higher than that of surrounding cells; therefore, these cells intrude between the neighboring cells penetrating the middle lamella. The review considers the classical example of intrusive growth, e.g., elongation of sclerenchyma fibers when the cells achieve the length of several centimeters. We sum the published results of investigations of plant fiber intrusive growth and present some features of intrusive growth characterized by the authors for flax (Linum usitatissimum L.) and hemp (Cannabis sativa L.) fibers. The following characteristics of intrusive growth are considered: its rate and duration, relationship with the growth rate of surrounding cells, the type of cell elongation, peculiarities of the fiber primary cell wall structure, fibers as multinucleate cells, and also the control of intrusive growth. Genes, which expression is sharply reduced at suppression of intrusive growth, are also considered. Arguments for separation of cell elongation and secondary cell wall formation in phloem fibers and also data indicating diffuse type of cell enlargement during intrusive growth are presented
Production of antihydrogen in relativistic collisions
No full textThe production of antihydrogen atoms Full-size image (<1 K) has been observed by the PS210 collaboration at LEAR. The method assumes an e+e− -pair creation via the two-photon mechanism in the antiproton-nucleus interaction. Neutral H̄0 atoms are identified by a unique sequence of characteristics. The typical signature of eleven produced antihydrogen atoms is reported including possibly 2±1 background signals. The observed yield agrees with theoretical predictions
Production of antihydrogen
No full textResults are presented for a measurement for the production of the antihydrogen atom , the simplest atomic bound state of antimatter. A method has been used by the PS210 collaboration at LEAR which assumes that the production of is predominantly mediated by the e+e−-pair creation via the two-photon mechanism in the antiproton-nucleus interaction. Neutral atoms are identified by a unique sequence of characteristics. In principle is well suited for investigations of fundamental CPT violation studies under different forces, however, in our investigations we concentrate on the production of this antimatter object, since so far it has never been observed before. The production of 11 antihydrogen atoms is reported including possibly 2±1 background signals, the observed yield agrees with theoretical predictions
Observation of antihydrogen production in flight at CERN
No full textThe observation of the production of antihydrogen atoms \overlineH 0\equiv\barpe+, the simplest atomic bound state of antimatter, is presented. A method has been used by the PS210 collaboration at LEAR which assumes that the production of is predominantly mediated by the e(+)e(-) pair creation via the two-photon mechanism in the antiproton-nucleus interaction. Neutral atoms are indentified by a unique sequence of characteristics. In principle is well suited for investigations of fundamental CPT violation studies under different forces, however, in our investigations we concentrate on the production of this antimatter object, since so far it had not been observed. The production of eleven antihydrogen atoms is reported including possibly 2 +/- 1 background signals, the observed yield agrees with theoretical predictions
Causes and impacts of changes in the stratospheric meridional circulation in a chemistry-climate model
No full textThe climate of the stratosphere is known to be subject to long-term changes induced
by anthropogenic emissions of long-lived greenhouse gases (GHGs) as well as by emissions
of ozone depleting substances (primarily chlorofluorocarbons, CFCs). Enhanced
concentrations of CFCs have led to strong ozone depletion over the last decades. Thanks
to the Montreal Protocol and its amendments and adjustments, the stratospheric halogen
loading is expected to retreat again in the future. Emissions of GHGs, on the
other hand, are not yet controlled sufficiently, and concentrations of GHGs are projected
to increase further in the future. The effects of enhanced GHG concentrations
on the stratosphere include decreasing temperatures, as well as changes in the dynamical
balances and interactions with the troposphere, and thus changes in the large-scale
circulation. In particular, the stratospheric meridional circulation is projected to be
subject to changes. These GHG-induced changes will also affect stratospheric ozone
chemistry and transport of ozone. Therefore, the expected recovery of ozone due to decreasing
CFC concentrations will coincide with alterations of the ozone layer by climate
change.
This study aims to diagnose and explain long-term changes in the stratospheric
meridional circulation using the chemistry-climate model E39CA. The dynamical causes
for these changes are identified, and the impact of changes in the meridional circulation
on the future development of ozone is quantified.
In a changing climate, the meridional circulation is found to strengthen in the tropical
lower stratosphere. In particular, tropical upwelling in the lowermost stratosphere
intensifies at a rate of about 3% per decade over the analysed period of 1960 to 2049.
This enhanced upwelling is balanced by downwelling in the subtropics at latitudes
around 20-40°N/S. The increase in tropical upwelling can be explained by stronger local
forcing by large scale waves. It is shown that enhanced tropical upwelling is driven
by processes induced by increases in tropical sea surface temperatures (SSTs). Higher
tropical SSTs cause both a) a strengthening of the subtropical jets and b) modifications
of deep convection, leading to changes in the strength and location of latent heat release.
While the former (a) can modify wave propagation and dissipation, the latter (b)
affects tropical wave generation. Evidence is presented that the dominating mechanism
leading to enhanced vertical wave propagation into the lower stratosphere is an upward
shift of the easterly shear zone due to the strengthening and upward and equatorward
shifts of the subtropical jets.
In addition to the increase in tropical upwelling caused by climate change, the
changes in CFC concentrations also affect the dynamical forcing of the meridional circulation.
The CFC-induced depletion of ozone in the past has led to changes in the
background wind field in the southern hemisphere in summer, which cause enhanced
wave propagation into the middle stratosphere and thus a strengthening of the meridional
circulation. This effect is reversed in the future, when CFC concentrations decline.
The future development of ozone is found to be dominated by changes in local
chemistry in most regions of the stratosphere. Both decreasing CFC concentrations and
stratospheric cooling due to enhanced GHG concentrations lead to less efficient ozone
destruction, and thus increasing ozone concentrations. However, changes in transport
of ozone due to the strengthening of the meridional circulation play an important role in
the tropical lower stratosphere, where ozone concentrations decrease due to more export
of ozone. Furthermore, it is found that the chemically induced positive ozone trend
in southern high latitudes in the future is counteracted by decreased ozone transport
from middle to high latitudes. This decrease in transport is due to the weakening of
the meridional circulation in the southern hemisphere in summer, which, in turn, is
induced by ozone changes
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