The main role of stems is to elevate and support the photosynthetic organs (leaves) as well as the reproductive organs (flowers). But in some plants the stem does other things for the plant. For instance, the potato tuber that we eat comes from the ground but is not a root; it’s an underground stem that acts as a storage organ for the plant. The “eyes” of a potato are depressions containing axillary buds—in other words, a sprouting potato is just a branching stem (Figure 33.15A). Many desert plants have enlarged, water-retaining stems (Figure 33.15B). The runners of strawberry plants are horizontal stems that develop adventitious roots some distance from the main stem (Figure 33.15C); if the links between the rooted portions are broken, independent plants can develop on each side of the break—a form of vegetative (asexual) reproduction (see Key Concept 37.3).
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.