Stem Modifications

Stem modifications, either aboveground, underground, or aerial, enable plants to survive in particular habitats and environments.

Learning Objective

Explain the reasons for stem modifications

Key Points

Modified stems that grow horizontally underground are either rhizomes, from which vertical shoots grow, or fleshier, food-storing corms.
New plants can arise from the nodes of stolons and runners (an aboveground stolon): stems that run parallel to the ground, or just below the surface.
Potatoes are examples of tubers: the swollen ends of stolons that may store starch.
The stem modification that has enlarged fleshy leaves emerging from the stem or surrounding the base of the stem is called a bulb; it is also used to store food.
Aerial modifications of stems include tendrils, thorns, bulbils, and cladodes..

Terms

cladode
green branches of limited growth which have taken up the functions of photosynthesis
tuber
a fleshy, thickened, underground stem of a plant, usually containing stored starch, as for example a potato or arrowroot
stolon
a shoot that grows along the ground and produces roots at its nodes; a runner
thorn
a sharp, protective spine of a plant
tendril
a thin, spirally-coiling stem that attaches a plant to its support
bulb
the bulb-shaped root portion of a plant such as a tulip, from which the rest of the plant may be regrown
bulbil
a bulb-shaped bud in the place of a flower or in a leaf axil
corm
a short, vertical, swollen underground stem of a plant that serves as a storage organ to enable the plant to survive winter or other adverse conditions such as drought
rhizome
a horizontal underground stem of some plants that sends out roots and shoots from its nodes

Full Text

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment. A rhizome is a modified stem that grows horizontally underground; it has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato. Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the "eyes" on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris .

Stem modifications

Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger (Zingiber officinale) rhizomes, (b) a carrion flower (Amorphophallus titanum) corm (c) Rhodes grass (Chloris gayana) stolons, (d) strawberry (Fragaria ananassa) runners, (e) potato (Solanum tuberosum) tubers, and (f) red onion (Allium) bulbs.

Modifications to the aerial stems, vegetative buds, and floral buds of plants perform functions such as climbing, protection, and synthesis of food vegetative propagation . Aerial modifications of stems include the following:

Aerial modifications of stems

Found in southeastern United States, (a) buckwheat vine (Brunnichia ovata) is a weedy plant that climbs with the aid of tendrils. This one is shown climbing up a wooden stake. (b) Thorns are modified branches.

Tendrils are slender, twining strands that enable a plant (like the buckwheat vine) to seek support by climbing on other surfaces. These may develop from either the axillary bud or the terminal bud of the stem.
Thorns are modified branches appearing as hard, woody, sharp outgrowths that protect the plant; common examples include roses, osage orange, and devil's walking stick.
Bulbils are axillary buds that have become fleshy and rounded due to storage of food. They become detached from the plant, fall on ground and develop into a new plant.
Cladodes are green branches of limited growth (usually one internode long) which have taken up the functions of photosynthesis.

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