, thorns (found in Citrus and Bougainvillea) and tendrils (found in Cucurbita and cucumber) are modifications of the stem (specifically axillary buds). The flattened, chlorophyll-containing structure of Opuntia is also a modified stem (phylloclade) adapted for photosynthesis in arid regions. The pitcher of Nepenthes is a modification of the leaf, not the stem.