Which of the following parts is not used by the plants for excretion?

Plants use various organs for excretion, including leaves, stems, and hydathodes. Leaves are a primary site for transpiration, where excess water and waste gases, such as carbon dioxide and oxygen, are removed from the plant. The stem also transports waste products from the plant to the leaves for removal. Hydathodes, which are specialized pores found at the tips of leaf veins, can also release excess water and waste. 

However, flowers are not used for excretion; instead, they are reproductive organs used in the process of pollination and fertilization.

Therefore, the correct answer is the flower.