Pine, because it can grow in poor, rocky soil, and stays green even in the worst of the winter, symbolizes survival through difficult circumstances.  Plum, because it blooms in winter and has delicate pure white blossoms, stands for both the purity of the scholar as well as beauty amid harsh conditions.  Plum, bamboo, and pine taken together evoke the Confucian virtue of maintaining one's integrity even in the most adverse conditions.