Figure 33.15 Modified Stems (A) A potato is a modified stem called a tuber; the sprouts that grow from its “eyes” are shoots, not roots. (B) The stem of this barrel cactus is enlarged to store water. Its highly modified leaves serve as thorny spines. Most of this plant’s photosynthesis occurs in the stem. (C) The runners of strawberry are horizontal stems that produce roots and shoots at intervals. Rooted portions of the plant can live independently if the runner is cut.