Plant Stems: Structure and Function

• Stems are the main structural axis of a plant and support leaves, flowers, and fruits.
• Stems are conduits for water, minerals, and nutrients.
• Stems connect roots to the rest of the plant through the vascular system.
• Stems play a role in storage, photosynthesis, and vegetative propagation.
• Stems grow through apical (primary) meristem, and in woody plants, lateral (secondary) meristem.

External Structure of a Stem

• Nodes are the points on the stem where leaves or branches grow.
• Internodes are the distances between nodes and elongate during growth.
• Axillary buds are found in the angle between the leaf and stem.
  • Axillary buds can develop into branches or flowers.
• Terminal buds are located at the stem tip and are responsible for primary growth.
• Leaf scars mark where leaves were attached and vary in shape.
• Lenticels are pores on the stem surface for gas exchange.
• Bud scales protect dormant buds from environmental damage.
• Stipules are small leaf-like structures at the leaf base, varying in function and presence.
  • Stipules protect young, developing leaves/buds before they emerge fully.

Woody vs. Herbaceous Plants

• Woody plants produce wood as their structural tissue.
  • They have strong stems covered with bark.
  • Woody plants are the tallest and largest plants on Earth and are mainly perennials.
• Herbaceous plants do not have persistent woody stems above ground
  • They have flexible and green stems.
  • Herbaceous plants are comparatively short and small, and can be annuals, biennials, or perennials.

Internal Structure of Monocot and Dicot Stems

• Monocot (herbaceous) stems have primary growth occurring in the apical meristem (length).
  • They usually lack secondary growth.
• Dicot (woody) stems have both primary and secondary growth.
  • Secondary growth occurs in the lateral meristems, increasing the stem diameter.

Monocot vs. Dicot Stem Characteristics

• Monocot
  • Epidermis with cuticle
  • Hypodermis made of sclerenchyma
  • Undifferentiated ground tissue
  • Vascular bundles are numerous and irregularly scattered, with absent bundle caps and present bundle sheaths.
  • Xylem vessels have only two protoxylem in each bundle
• Dicot
  • Epidermis with trichome and cuticle
  • Hypodermis made of collenchyma
  • Ground tissue differentiated into cortex, medulla rays, and pith
  • Vascular bundles are eight in number, arranged in a ring, with present bundle caps and absent bundle sheaths.
  • Xylem vessels have many protoxylem and metaxylem in each bundle.

Protoxylem and Metaxylem

• Protoxylem is the primary xylem tissue that develops first in a plant.
  • It has smaller cells with thinner cell walls.
• Metaxylem is the secondary xylem tissue that forms after protoxylem.
  • It has larger, more mature cells with thicker cell walls.

Secondary Growth in Stems

• Secondary growth occurs in woody eudicots and conifers, produced by the vascular cambium.
• Lateral meristem produces secondary xylem (wood) on the inside and secondary phloem (bark) on the outside, located between the primary xylem and primary phloem.

Formation of Vascular Cambium

• Within the primary vascular bundle, cells differentiate and form a cylindrical layer of meristematic tissue called the vascular cambium.
• Secondary xylem formation occurs as the vascular cambium produces secondary xylem cells inwardly toward the pith.
  • These cells differentiate into vessel elements, tracheids, fibers, and parenchyma cells, contributing to wood growth.
• Secondary phloem formation: the vascular cambium produces secondary phloem cells outwardly toward the cortex.
  • These cells differentiate into sieve tube elements, companion cells, fibers, and parenchyma cells, contributing to bark growth.

Formation of Cork Cambium and Cells

• The outermost layer of the stem cortex undergoes dedifferentiation to form a new meristematic tissue called cork cambium (phellogen).
• Cork cambium continues to divide, forming several layers of cork cells to form the periderm.
  • Periderm replaces the epidermis as the protective cover of the stem.

Maturation and Sloughing Off of Epidermis

• As the periderm develops, the epidermis, along with stomata and trichomes, sloughs off.
• The periderm becomes the new protective layer of the stem, aiding in protection against various environmental stresses.

Wood and Bark

• Wood is known as secondary xylem, a type of tissue found in the inner layers of the stem.
  • Wood is primarily composed of dead cells, like vessel elements, tracheids, fibers and parenchyma cells.
• Bark refers to the protective outer covering of the stem, arising from the secondary phloem.
  • The outermost layer of bark is the cork or periderm, produced by the cork cambium (phellogen).

Rays

• Rays are one of the ribbons of tissue extending radially from the pith.
  • Rays store and transport food horizontally within the tree.

Stem Modifications

• Rhizomes are horizontal underground stems that serve as storage organs and for sexual reproduction (e.g., ginger, iris).
• Tubers are the thickened ends of a rhizome that are fleshy and enlarged for food storage (e.g., potato).
• Bulbs are rounded, fleshy underground buds consisting of a short stem with fleshy leaves (e.g., onion).
• Corms are short, thickened underground stems specialized for food storage and asexual reproduction (e.g., crocus).
• Stolons are aerial horizontal stems with long internodes.
  • Stolons form buds that develop into separate plants (e.g., strawberry).
• Tendrils are slender, threadlike appendages of a climbing plant.
  • Tendrils often grow in a spiral form to stretch out and twine around suitable support (e.g., ampalaya).
• Cladophylls are shoot systems where leaves do not develop.
  • The stems become flattened and photosynthetic (e.g., cactus).