Fix the mistake: Plant Structure

Leaf Structure and Stomata

Plant leaves are not simply thin green sheets. They are precise factories for photosynthesis and gas exchange.

Basic Knowledge

Cross-sectional structure of a leaf (from the top):
① Epidermis — A single layer of cells, with a cuticle layer on the outside (prevents evaporation)
② Palisade tissue — Many chloroplasts arranged closely together. The main site of photosynthesis
③ Spongy tissue — Spaces between cells. Pathways for gases
④ Leaf veins — Vascular bundles (xylem and phloem)
⑤ Lower epidermis — Contains many stomata

A stoma (stoma, plural stomata) is a small pore surrounded by two guard cells. It is an entrance and exit for oxygen, carbon dioxide, and water vapor. In many plants, stomata are distributed mainly on the underside of leaves. Guard cells contain chloroplasts and open and close the stomata depending on light and water conditions.

Important Terms
Chloroplast (chloroplast)（A green cell organelle that carries out photosynthesis. Contains chlorophyll）
Palisade tissue（An orderly layer of cells on the upper side of a leaf. The main site of photosynthesis）
Spongy tissue（An irregular layer of cells on the underside of a leaf. Gases pass through it）
Stoma (stoma)（A small pore in the leaf epidermis. An entrance and exit for gases）
Guard cell (guard cell)（Two cells surrounding a stoma. They open and close the stoma）
Cuticle layer（A waxy layer covering the leaf epidermis. Prevents water from evaporating）

Further Study (Background and Significance)

Palisade tissue is densely packed on the upper side of a leaf in order to receive light efficiently. Since light enters from the upper side, it is reasonable to arrange cells containing many chloroplasts there.
Stomata are mainly on the underside to avoid direct sunlight and drying. To prevent excessive water loss through stomata, plants sense temperature, sunlight, and soil moisture and open or close their stomata.
Guard cells are crescent-shaped and have thick inner cell walls. When water enters, their outer sides swell and the stoma opens. This is a precise mechanism by which plants respond to their environment even though they cannot move.
Aquatic plants (such as Canadian waterweed) show diverse forms suited to their habitats: their stomata may be on the upper side, or the stomata themselves may have degenerated.

Key Points
  • Leaf structure (top → bottom): epidermis, palisade tissue, spongy tissue, lower epidermis
  • Palisade tissue contains the most chloroplasts
  • A stoma is a pore surrounded by two guard cells
  • Stomata are mainly on the underside of leaves
  • Stomata are entrances and exits for gases and water vapor
  • Guard cells contain chloroplasts and open and close stomata
  • The cuticle layer prevents water from evaporating

Points to Note (Easily Confused)

① Do not reverse palisade tissue (upper side, dense) and spongy tissue (underside, with spaces). ② A stoma (pore) and guard cells (cells surrounding the pore) are different. ③ Stomata are generally more numerous on the underside of leaves, although exceptions occur depending on the plant species. ④ The cuticle layer is a waxy substance on the outside of the epidermis, not a cell.

Practice

  1. Which tissue in a leaf carries out the most photosynthesis?
  2. What are the two cells surrounding a stoma called?
  3. Are stomata generally more numerous on the upper or underside of a leaf?

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