Plant Epidermis: Deposits, Cells, and Trichome Secretion

Posted on Mar 12, 2025 in Biology

Plant Epidermis: Deposits, Cells, and Secretion Mechanisms

Cuticular and Epidermal Deposits

In epidermal cells, various salt deposits manifest as crystals. These deposits are located both on the cuticle surface and permeating it, even within the cell wall itself. Many plants exhibit significant mineralization of the cell wall, with silica joining other salt deposits. These salts distribute evenly between cellulose and cutin. Calcium carbonate mineralization appears coarser than silica, with a granular texture. It typically deposits on the surface, sometimes forming a considerable thickness, as seen in certain calcareous plants.

Some epidermal cells in the leaves of Urticaceae and Moraceae feature a unique calcium carbonate deposit: calcified spherical concretions, grouped in clusters called cystoliths. These are found inside specialized epidermal cells, larger than surrounding ones, hanging from a superficial cellulose filament, around which the calcium carbonate deposits. Another type of mineralization involves calcium oxalate, which partially fills the cell wall (sometimes lignified), forming crystallites embedded within the wall’s thickness. Other epidermal cells may contain deposits like resins and oils. Some plant groups develop mucilaginous epidermal cell walls.

Specialized Epidermal Cells

  • Bulliform cells: Found in the epidermis of grass leaves and other monocots. Characterized by a large vacuole with high water content, they are significantly larger than other epidermal cells and lack chloroplasts. Their thin cell wall consists only of a primary wall covered by a cuticle on the outer surface. They are generally distributed in bands parallel to the veins, in the depressions of inter-vein areas.
  • Siliceous cells: Characteristic of grasses, containing silica bodies that can be circular, elliptical, flared, or saddle-shaped. These bodies are membrane-bound, suggesting they reside within a vacuole.
  • Cork cells:Often found adjacent to siliceous cells. Their cell wall is impregnated with suberin, and their vacuole frequently contains reserve substances.

Trichome Secretion Mechanisms

Plant trichomes release secretions through three primary mechanisms:

  • Via channels or pores: Secretion passes through the plasma membrane and then the cell wall. Similar to animal glands, some secretions (small ions, molecules, and lipids) diffuse passively through the plasma membrane due to cellular pressure. Other secretions (proteins, glycoproteins, polysaccharides) travel in vesicles from the Golgi apparatus to the plasma membrane, where they fuse. Once through the cell wall, the secretion is extruded via channels or pores in the cuticle. This is observed in active hydathodes, some colleters, mucilaginous glands, and osmophores.
  • By cuticle rupture: Secretion is formed and transported as in the previous case. However, after crossing the cell wall, it accumulates between the wall and the cuticle, causing the cuticle to detach. Slight touch or heat can rupture the cuticle, releasing the contents. This occurs in nectaries, some mucilaginous glands, and colleters not included in the previous group.
  • Through secretory cell death: Some secretions are released only upon the death of the cells containing them. This is seen in glands secreting oils, where the cytoplasm undergoes progressive degeneration, becoming runny. In vesicular hairs secreting salt, the secretion is released when the leaf ages and the secreting cells dry out.