Antagonistic interactive effects of exogenous calcium ions and parasitic Cuscuta australis on the morphology and structure of Alternanthera philoxeroides stems Effect of exogenous calcium ions on the parasitic effects of Cuscuta australis on the invasive plant Alternanthera philoxeroides Exogenous Ca2+ Regulation of Pollen Tube Growth and Division of Generative Nucleus in Nicotiana tabacum