Abstract:The effect of soil drought stress on the ultramicrostructure of mesophyll cells was studied and observed by SEM using the potted seedlings of the following trees in the greenhouse, i.e. Quercus acutissima Carruth, Dalbergia hupeana Hance, Pistacia chinensis, Pinus elliottii and Celtis sinesis Pers. The experiment results showed that under the condition of well water state, the organelles of mesophyll cells keep integrity. Under the condition of light drought stress, the ultramicrostructure of P.elliottii was not damaged. The mitochondria of Q.acutissma Carruth remained normal, but the chloroplasts expanded. For C.sinesis Pers, plasmolyses occurred and the mitochondria and chloroplasts were damaged. Under the condition of heavy drought stress, the mitochondria of P.elliottii and Q.acutissima Carruth were degraded, and the chloroplasts were not adhered to the cell walls. For P.chinensis and D.hupeana Hance, plasmolyses occurred and the mitochondria and chloroplasts were severely damaged. The cells of C.sinesis Pers were damaged most severely. The results showed that the drought resistance of these five plants could be divided into three degrees: P.elliottii and Q.acutissima Carruth have strong drought resistance, P.chinensis and D.hupeana Hance have moderate drought resistance while C.sinesis Pers has the weakest drought resistance.