Final answer:
The cactus, particularly the pear, has a flattened, photosynthetic stem which helps it conserve water in arid environments.
Step-by-step explanation:
Modified organs are part of survival strategies of plants. The plant that has a flattened, photosynthetic stem that could be mistaken for a leaf is cactus. Specifically, the pear cactus (Opuntia sp.) has evolved in such a way that its leaves are modified into spines to reduce water loss, while photosynthesis primarily takes place in the stem. This adaptation is crucial as it allows the cactus to survive in hot, arid climates by conserving water.
Stems that harbor chlorophyll which gives them a green color. Often plants with photosynthetic stems drop their leaves (are deciduous) at a certain time of year and whatever photosynthesis that takes place at that time is within the stem chlorophyll. Usually plants with photosynthetic stems grow under environmental stress at certain times of the year, e.g., extremely dry periods during which considerable evapotranspiration would take place if the leaves were retained.