Internal structure of leaf

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A. General Features

Leaves of most dicot plants are dorsiventral (oriented horizontally, with differentiated mesophyll) where as those of monocots are isobilateral (oriented vertically, mesophyll undifferentiated) as seen in transverse section shows the following features;

Epidermis: the upper and lower surfaces of the leaf are covered by the upper and lower epidermis, single layered cells, lack chloroplasts, some epidermal cells form stomata which help in exchange of gases for photosynthesis, respiration and evaporation of water vapour during transpiration.

Mesophyll: the entire tissue between the upper and lower epidermis, containing parenchyma (Chlorenchyma) and is responsible for carrying out photosynthesis and divided into two regions; palisade and spongy cells. Palisade cells: occurs beneath the upper epidermis, abundant chloroplasts so upper surface of the leaf appears dark green, cells are arranged parallel and radially elongated. Spongy cells: occurs below the palisade cells with loosely arranged and irregularly shaped cells with intercellular spaces, contain fewer chloroplasts and helping in the distribution and diffusion of gases.

Vascular Bundles: They are conjoint, collateral, endarch and closed. The xylem is situated towards the upper epidermis (adaxial surface) and phloem lies towards the lower epidermis (abaxial surface). Each vascular bundle is surrounded by a layer of thick-walled cells arranged compactly and known as the bundle sheath.

Image showing T.S. of leaf; (a) Dicot (b) Monocot.

Image Showing T.S. Of Leaf.

Image showing T.S. of leaf; (a) Dicot (b) Monocot.

Structure of Stomata

Each stoma is surrounded by two guard cells. The kidney-shaped guard cells contain chloroplasts. A respiratory cavity or chamber is found under each stoma. The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch in the guard cells.

Distribution of Stomata

Table Showing Distribution of Stomata.
Table showing distribution of stomata.

Plants

Stomatal characters

Examples

Dicots

Guard cells semicircular, present on lower surface of leaf

Mango, Neem

Monocots

Guard cells dumbbell shaped, equally distributed on both the surfaces

Maize

Xerophytes

To reduce transpiration- occur only on lower surface, are less in number, may be sunken

Nerium

Hydrophyte

-With floating leaves

-With submerged leaves

Occur only on upper surface

Stomata absent

Lotus

Hydrilla

Difference between Internal Structure of Dicot and Monocot Leaf

Table Showing Difference between Dicot and Monocot Leaf.
Table showing difference between dicot and monocot leaf.

Tissue

Dicot leaf

Monocot leaf

Cuticle

Cuticle thick at upper epidermis and thin at lower epidermis

Uniform cuticle on both the surface

Stomata

more on lower surface

Equal number of stomata on either side

Mesophyll

differentiated into palisade parenchyma and spongy parenchyma

differentiated into palisade and spongy parenchyma

Bulliform cells

Absent

Present

Vascular system

in the form of network, vascular bundle in mid rib region is large, rest small

in rows, vascular bundle generally of same size

B. Special Features

Bulliform cells: In monocots, a few large, empty and colourless cells are present at intervals on the upper surface is called the Bulliform cells are found most commonly seen in the upper epidermis and in the rolling and unrolling of leaves in response to their water status.

Hairs: Present on leaves of plants which are growing in dry conditions. They check the rate of transpiration and protect the leaf from high temperature.

Hydathodes (water stomata): Loss of water through water stomata (Hydathodes) is called guttation. Guttation mainly occurs when transpiration is difficult due to high air humidity or when the plant is placed in a watery soil.

Image showing hydathode and stomata.

Image Showing Hydathode and Stomata.

Image showing hydathode and stomata.

Difference between Stomata and Hydathode

Table Showing Difference between Stomata and Hydathode.
Table showing difference between stomata and hydathode.

Characters

Hydathode

Stomata

Size

Large

Small

Location

On the tips of young leaves at vein endings

In epidermis of leaves, stems and fruits.

Structure

always open

opening and closing by guard cells

Loss of water

loss of water in the liquid form

loss of water in the gaseous form

Occurrence

Found in plants of humid areas

In plants occurring in all climates

process

Guttation

Transpiration

surrounded by

ring of cuticularised achlorophyllous cells

pairs of chlorophyllous guard cells