The Effects of Cr6+‘s Pollution on the Ultrastructure of Hydrocharis dubia and Cell Membrane of H.verticillata,Brasenia schreberi,Trapa bispinosa Effect of PulsedMagnetic Field on the Growth, Physiolog ica l andBiochem ica l Character istics of P inus m asson iana Ca llus Effects of Soil Cu2+ Stress on Some Physiological Characteristics of Italian Ryegrass