Plant Anatomy: Glands, Roots, and Specialized Structures
Plant Glands: Types and Secretions
Glands: External Secretion of Mucilage Cells: Cells with large nuclei, thick cell walls, numerous dictyosomes, and vesicles in the periphery. They contain mucilage and exhibit granulocrine secretion.
Glandular Salt Structures: Plants that live in saline soils, known as halophytes (e.g., Atriplex), have an epidermis covered with vesicular hairs that accumulate salts. In Caragana, salts are stored in vacuoles. When the cell collapses, the liquid content evaporates, leaving a powdery layer of salts on the plant, probably through eccrine secretion.
Colleters: These are trichomes or multicellular emergences with a sticky secretion. They have a body and a foot, which may be missing. The cells of the shaft are arranged in a radial epidermal cylinder. They secrete a mixture of terpenes and mucilage to sheath buds, demonstrating merocrine secretion.
Glands of Carnivorous Plants: These plants have complex digestive glands that secrete enzymes to digest nutrients. The epidermis contains two types of glands:
- Capitate glands (pedicellate): Secrete mucus to adhere to the prey.
- Sessile glands: Secrete proteolytic enzymes.
Both types are at the base of the endodermal cells, which are transfer cells.
Nectaries: These glands secrete nectar to attract insects and birds. Nectar contains sugars (sucrose, fructose), simple sugars, and polysaccharides like bushy and melibiose. There are two types:
- Non-structural: The tissue is not different from the adjacent tissue.
- Structural: The tissue differs from the adjacent tissue.
Nectaries can be accessible or constituted by trichomes, epidermal or parenchymal tissue, and may or may not be vascularized. If vascularized, the nectar exits through the cuticle. If parenchymatous, it exits through non-functional stomata.
Position of Nectaries:
- Extrafloral: Attract animals.
- Floral: Attract pollinators; located in the innermost whorls (calyx, petals, stamens).
Osmophores: These produce odorous substances (essential oils) that create the smell of flowers. Their cells are rich in starch, and they are located between the adaxial vascular bundles and the epidermis.
Hydathodes: These structures expel water from the inside of the leaf blade to the surface through a process called guttation. This occurs when the plant is in a position that favors minimal perspiration and absorption of minerals (e.g., plants growing in wet soil). Increased pressure in the xylem forces water out through the hydathodes.
Root Anatomy: Structure and Function
Roots specialize in:
- Fixation of the plant to the substrate.
- Absorption of water and dissolved minerals.
- Transport of water and solutes.
- Storage.
- CO2 transport in plants whose leaves lack stomata.
Origin: The first root of a plant originates from the root apical meristem.
Structure
Epidermis or Rhizodermis (Dermal System): This is a single-layered, absorbent tissue. The cells are elongated without a cuticle. In aerial roots of epiphytic plants, some monocots have a multi-layered epidermis called a velamen. This is formed by dead cells and provides mechanical protection and reduces water loss from the bark. Root hairs originate from tubular extensions of epidermal cells called trichoblasts, increasing the absorptive surface area.
Cortex (Fundamental System): This region is located between the rhizodermis and the vascular cylinder. It is composed of homogeneous parenchymal cells or different cell types (idioblasts: tanniferous, crystal). The parenchyma may differentiate into:
- Aerenchyma (in wet environments).
- Storage parenchyma (e.g., sweet potato, carrot).
- Secretory cavities.
It may contain sclerenchyma in the more external layers. It also presents:
- Exodermis: One or more peripheral layers of the cortex beneath the epidermis. It may have Casparian bands and suberin lamellae covering the cell wall, similar to the endodermis in structure and biochemistry.
- Endodermis: The innermost layer of the cortex. It acts as a barrier to apoplastic movement, allowing selective passage of minerals and water through the plasma membrane to the vascular cylinder. The primary endodermis has Casparian bands of suberin around radial and transverse walls. In the secondary state, a uniform layer of suberin is deposited on the inner wall. In the tertiary state, a layer of cellulose, which may be lignified, is deposited on the suberin layer.
Vascular Cylinder: This is composed of nonvascular parenchyma cells. It contains:
- Pericycle: Contains secretory schizogenous ducts and sclerenchyma. It is interrupted by the protoxylem and gives rise to the vascular cambium and cork cambium in lateral roots.
- Xylem: Forms the center with projections towards the pericycle, resulting in diarch, triarch, or polyarch root formation. It alternates with bands of phloem. Both have centripetal maturation, with protoxylem and protophloem in peripheral positions in the cylinder (exarch xylem). There is no differentiation in the root xylem, which is formed by parenchyma and sclerenchyma.