Special Stems

Stems of most plants transport water/ minerals through the plant and strengthen the plant; specialized stems perform additional functions (see below)

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Rhizomes

Rhizomes are underground stems that grow horizontally
- This growth, in which plants grow transversely to gravity is called diagravitropic
Frequently, rhizomes are indeterminate or perennial portions of the plant
Rhizomes send up determinate shoots, which photosynthesize and reproduce, but then die-back after time
e.g. many herbaceous perennials exhibit rhizomes, such as bamboo, daisies, and ferns
One of the largest plants on Earth is a patch of seagrass off the western coast of Australia
- The patch is a large clone of Posidonia australis, which, over 4,500 years, has spread through rhizomes about 200 sq km or 77 sq miles (just over three times the size of Manhattan).

Above: The rhizome and upright shoots of Ginger (Zingiber)

Left: A very large patch of seagrass (Posidonia australis) using rhizomes to spread over 200 sq km of underwater seafloor, off the western coast of Australia.

Stolons

Creeping stems that grow horizontally above the ground
- They are similar to rhizomes, which creep horizontally underground
- This growth, in which plants grow transversely to gravity is called diagravitropic
As stolons grow, they set down roots periodically to create plantlets
Sometimes stolons called runners
e.g. Strawberry (Fragaria) and Hawkweed (Hieracium) exhibit stolons

Above: The stolons of Strawberry (Fragaria)

Water-storage stems

Stems modified to store excess amounts of water
The anatomy of these plants have extra parenchyma with large vacuoles for storing water
This adaptation can be found in arid areas (e.g. deserts), dry microenvironments (e.g. epiphytic cacti), and also salt marshes (e.g. Salicornia)
e.g. Cactus family (Cactaceae) and Baobob trees are good examples of water storage stems

Above: Water storage stems of Barrel cactus (Echinocactus)

Above: Water storage stems of Baobab trees (Adansonia)

Above: Salicornia, which is a stem succulent and a halophyte inhabiting salt marshes and mangroves

Above: Bottle tree (Pachypodium) is a stem succulent

Above: Elephant palm (Beaucarnea recurvata), is also a stem succulent

Food-storage stems

Stems modified to store excess amounts of starch
- The anatomy of these plants have parenchyma with amyloplasts
Food-storage stems tend to be underground, serving as part of the rhizome system of the plant
Colloquially food-storage stems are referred to as tubers
- The axillary buds on these stems, which are referred to as "eyes", allow for regrowth
- n.b. some food-storage roots are also called tubers
These modified stems allow plants to regrow after a period of dormancy (e.g. winter); usually found in perennial plants
e.g. White potatoes (Solanum tuberosum), Cyclamen, and Yams (Dioscorea) exhibit food-storage stems

Above: Stem tubers of yam (Dioscorea alata)

Corms

Corms are bulb-like compressed storage stems
- They are usually found underground
Stem tissues exhibit both food-storage qualities as well as embryonic shoot tissues
- Most of a corm is stem tissues, with papery leaves on the outside, and roots on the bottom
e.g. Gladiolus and Taro (Alocasia) exhibit corms
Pseudobulbs in orchids are corm-like, but aboveground
n.b. Bulbs are similar in form and habit but are mostly composed of modified leaves

Above: The corm of Gladiolus

Thorns

Thorns are pointed and stunted branches, which are usually tough and woody (filled with sclerenchyma)
Thorns are modified stems or branches to protect against herbivory by large animals
Like other branches, thorns emerge from an axillary bud
- They should exhibit a leaf or leaf scar underneath the thorn
Some thorns exhibit branching, or multi-pronged thorns

Above: Thorn-like stems of firethorn (Pyracanthus). Note the leaves and axillary buds on the thorns.

Above: Thorns of hawthorn (Craetagus). Notice the horizontal leaf scar below the thorn

Stem Tendrils

Stem tendrils are modified stems that provide climbing support for the plant
They appear from an axillary bud, as a modified branch
e.g. Grape (Vitis), Virginia creeper (Parthenocissus), Passionflower (Passiflora) all exhibit stem tendrils
Leaf tendrils serve a similar function but are modified leaves. Leaf tendrils are more common in the plant world than stem tendrils

Above: Stem tendril of grape vine (Vitis)

Cladophylls or Cladodes or Phylloclades

Cladophylls ("stem leaves") are modified stems that appear leaf-like
Most plants that have cladophylls, have reduced leaves, therefore these modified stems function to conduct photosynthesis
e.g. Prickly-pear cacti (Opuntia), the "leaves" of both Butcher's broom (Ruscus), the umbrella pine (Sciadopitys), and especially the celery pines (Phyllocladus) as the scientific name signifies

Above: Cladophylls of Prickly-pear cactus (Opuntia)

Above: Cladophylls of the Butcher's broom (Ruscus)

Above: Cladophylls of the Umbrella pine (Sciadopitys)

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