Internal Structure of Leaf

Leaves of most dicot plants are dorsiventral (oriented horizontally, with differentiated mesophyll) where as those of monocots are isobilateral (oriented vertically, mesophyll undifferentiated).

Epidermis - Present on both upper and lower surface of leaf. Some epidermal cells give rise to guard cells that get arranged to form openings called stomata which help in exchange of gases for photosynthesis, respiration and evaporation of water vapour during transpiration. In some monocot leaves, some epidermal cells in upper epidermis become enlarged to form bulliform cells which lose water so that leaves become tubular to reduce transpiration on hot sunny days.

Mesophyll - Consists of chloroplast - containing parenchyma (chlorenchyma) and is responsible for carrying out photosynthesis. It is differentiated into palisade and spongy cells in dicot leaves. In monocot leaves, palisade tissue is lacking, thus, mesophyll has only spongy tissue.

Palisade cells - occur below upper epidermis in dicot leaf. Cells are radially elongated, compactly arranged. They possess abundant chloroplasts.

Spongy cells - Occur below the palisade cells in a dicot leaf. Cells irregular and loosely arranged. They contain fewer chloroplasts. They store gases in the inter cellular spaces.

Vascular Bundles - They are conjoint, collateral and closed. In each bundle, xylem is located on upper side (ventral) and phloem on lower side (dorsal). Most vascular bundles are surrounded by colourless parenchyma called bundle sheath or border parenchyma.

Structure of stomatal apparatus

In dicot leaves, stomatal apparatus is made up of two semi circular guard cells surrounding a pore-stoma. The guard cells contain chloroplasts and regulate the opening and closing of stomata. Stomatal pore opens into the inter cellular spaces (substomatal cavity) of mesophyll. The number, shape and distribution of stomata vary depending upon the plant whether it is xerophyte or mesophyte.

Bulliform Cells

• These are special type of cells (motor cells) found on upper leaf surface of some monocots (e.g. maize, bajra, jowar).
• They help the leaf to roll and unroll due to change in their turgidity.
• Leaf rolls when these cells lose water due to high rate of transpiration especially at Mid-day on hot sunny days.
• Thus, under dry conditions they help in reducing the loss of water vapour through stomata.

Hairs

• Hairs are present especially on leaves of plants growing in dry conditions. They check the rate of transpiration.
• They protect the leaf from bright sunlight, high temperature and air pollution.

Hydathodes (water stomata)

• These are specialized structures present in leaves of angiosperms (garden nasturtium) occurring in humid climate.
• Through these openings excretion of water and minerals plus simple organic compounds in liquid form (guttation) takes place. When water absorption by a plant is more and transpiration is less.