Name any plants in which the stem is modified into a leaf-like structure and leaves are modified into spines?

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Hint: Occasionally the stem gets modified into a flattened green structure to perform the function of leaves, (i.e., photosynthesis) by becoming a thick, fleshy, and flattened structure. This contains chlorophyll and stores food and is known as phylloclade. Leaves are modified into spines to reduce the surface area and minimize the rate of water loss by evaporation and transpiration. This modification protects the plant from grazing animals.

Complete answer:
The stem is modified into a leaf-like structure and leaves are modified into spines in cactus. The plant Cactus grows in the desert area. To avoid the loss of water in these arid and dry conditions, modifications occur in the stem and it changes into a leaf-like structure and leaves. They have been modified into sharp pointed thin structures called spines. These spines decrease the surface area for preventing the loss of water.

Cacti show several adaptations to conserve water. Almost all cacti are succulents, meaning they have thickened, fleshy parts that are adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this crucial process takes place. Extensive species of cacti have lost true leaves and conserve only spines, which are highly modified leaves. As well as defending against herbivores, spines help stave off water loss by decreasing airflow close to the cactus and providing some shade. In the absence of leaves, enlarged stems run photosynthesis.

Note: A cactus is a part of the plant family Cactaceae. It comprises about 127 genera with some 1750 known species belonging to order Caryophyllales. The word "cactus" was initially used by Theophrastus for a spiny plant whose identity is now not certain.