Comparative Study on Gas Exchange Characteristics of Populus euphratica and P.pruinosa in the Middle Reaches of Tarim River Comparative Study of Photosynthetic and Chlorophyll Fluorescence Characteristics of Populus euphratica and P. pruinosa Total DNA Extraction and RAPD System Optimization of Endangered Plant Populus euphratica and P.pruinosa Preliminary study on water parameters of Populus euphratica and P.pruinosa by using P-V curve