Leaves of plants helps in excretion in many ways. Excess of water in plants is discarded by transpiration and guttation that takes place through leaves. Carbon dioxide and oxygen that can be considered as waste products of respiration and photosynthesis respectively are excreted through stomata present on leaves.
Besides these leaves helps in excretion through a process called abscission. Abscission is falling off old and yellow leaves. By the help of this process plants get rid off toxins etc.
Many plants store waste materials in the vacuoles of mesophyll cells and epidermal cells. When old leaves fall, the waste materials are excreted along with the leaves.