Plant Secretory Structures: Types and Functions
This document details the fascinating world of plant secretory structures, focusing on the mechanisms of substance excretion and the release of metabolic products. We will differentiate between external and internal secretory tissues.
External Secretory Tissues
External secretory tissues include glandular trichomes and other epidermal appendages. They are found throughout the plant and play roles in protection and absorption. Secretions can occur via:
- Diffusion through the membrane, predominantly involving the plasma membrane and, in some cases, the Golgi apparatus. Secretions exit through channels or pores in the cuticle.
- Holocrine secretion, where substances are released through the death of secretory cells.
Types of Glandular Trichomes
- Hydathodes: These are specialized glands that excrete an aqueous solution through pores. They consist of a stalk and an oval head.
- Salt-secreting trichomes: Found in plants growing in saline environments, these trichomes have a vesicular stalk cell and a secretory apical cell with a large vacuole.
- Mucilage-secreting trichomes: These glands produce polysaccharides. Mucilage, with its high water content, is expelled through pores, traversing the cuticle. This is common in plants from arid climates.
- Osmophores: These structures release oils and terpenes, contributing to the plant’s scent. Secretions exit through cuticular pores.
- Colleters: These secrete a mixture of terpenes and mucilage. Secretions are released through the rupture of the cuticle or cuticular pores.
- Enzyme-secreting glands: Found in carnivorous plants, these glands secrete enzymes that digest prey. The products are then absorbed through cuticular pores.
- Stinging hairs: These multicellular structures have a basal foot and a unique secretory cell shaped like a bottle, containing an irritating substance within a vacuole.
- Nectaries: These structures, which can be floral or extrafloral, secrete nectar, a sugar-rich liquid derived from the phloem. Nectar is released through the rupture of unicellular hairs or specialized portions of the epidermis.
Internal Secretory Tissues
Internal secretory tissues accumulate substances within the plant. Examples include:
- Laticifers: These are specialized cells or tissues that contain latex.
- Extracellular spaces: These include spherical cavities and elongated ducts.
Resin Ducts
Resin ducts are common in conifers and can form in response to damage. They consist of a cavity surrounded by resin-producing cells. In Pinus, the cavity is surrounded by a layer of epithelial cells and a sheath of outer cells. The development of resin ducts involves cell growth or the formation of a hollow space through a schizogenous process. These cells contain plastids and starch.
- Primary resin ducts: These are not associated with the primary xylem and extend near the stem apex. They continue into the cortex region without connecting to the root. Each juvenile leaf has two resin ducts.
- Secondary resin ducts: These are found in both the vertical and horizontal phloem. Oleoresin, found near the cortex, is composed of two parts: turpentine (used as a solvent) and resin (used in varnishes and adhesives).
Other Internal Secretory Structures
- Mucilage canals: These are derived from parenchyma cells and are formed through a lysigenous process, where cells degenerate and release mucilage.
- Gum ducts (Gummosis): In this process, cambium cells form specialized parenchyma cells. Gum production begins in the center of the cell and spreads, eventually leading to the disintegration of the primary cell wall.
- Kino veins: These contain polyphenols and vary in length. They differentiate in the cambial region in response to injury. Traumatic parenchyma cells differentiate and disintegrate, increasing the kino content in the veins.
- Laticifers: These are organs that extend throughout the plant, including the stem and root. They contain latex, a whitish or orange emulsion of water and various substances. Laticifers are believed to play a role in plant protection.
Types of Laticifers
- Articulated laticifers: These originate from a single cell that elongates and may branch. They grow intrusively through tissues and are multinucleated.
- Non-articulated laticifers: These are formed by the union of several mononuclear cells that differentiate from parenchyma. They can be branched or unbranched.
Laticiferous Cells
Laticiferous cells are distinct from apical meristems. They have long, thick primary cell walls that are supple and living, with few pores. In articulated laticifers, latex is stored in the vacuole.