INTERNAL STRUCTURE OF YOUNG LEAVES

INTERNAL STRUCTURE OF LEAF

Generally leaves are divided into two categories - Dorsiventral leaves and isobilateral leaves. The differences in between them are as follows:

Dorsiventral or Bi-facial- Present at right angle to stem. Upper surface of leaf receives more sun light as compared to the lower surface, so there occur difference between internal structure of upper and lower surfaces.

Example - Dicots

Exceptions - Eucalyptus and Nerium

(leaves are isobilateral)

Iso-bilateral or Equifacial- Arranged parallel to stem. Both surfaces of leaf receive equal amount of sun light so no difference occurs between Internal structure of upper & lower surfaces.

Example: Monocots Exception - Lilium longiflorum (leaves are dorsiventral).

INTERNAL STRUCTURE OF DORSIVENTRAL LEAF

Cuticle is present on both surfaces but cuticle on upper surface is more thick.

In dorsiventral stomata are more on lower surface and stomata on upper surface are absent or less  in number.

The tissue between the upper & the lower epidermis is called the mesophyll. In dicot leaf, mesophyll is differentiated into palisade parenchyma (palisade mesophyll or palisade tissue) and spongy parenchyma (spongy mesophyll or spongy tissue).

Palisade tissue is situated towards the upper (adaxial or ventral) surface. It is made up of elongated cells. which are arranged vertically and parallel to each other and have more chloroplasts and a large vacuole.

Spongy tissue is situated towards lower (abaxial or dorsal) surface. The cells are oval or rounded and between the cells large air spaces/ air cavities are present.

INTERNAL STRUCTURE OF ISOBILATERAL LEAF

The thickness of cuticle is equal on both surfaces.

Usually stomata on both surfaces are equal in number.

Mesophyll is not differentiated into palisade and spongy tissues in isobilateral leases. Mesophyll cells have only a few intercellular spaces.

Note: 1. In isobilateral leaf, two distinct patches of sclerenchyma are present above and below the large vascular bundle and extend up to the upper and lower epidermal layers. These are called bundle sheath extensions.

2. In dorsiventral leaf, two distinct patches of parenchyma (mainly)/collenchyma are present above and below the large vascular bundle and extend up to the upper and lower epidermal layers. These are called bundle sheath extensions. Chloroplasts are absent in bundle sheath extensions.

VASCULAR BUNDLES OF LEAVES :

Similar types of vascular bundles are found in both dorsiventral and isobilateral leaves. Vascular bundles of leaves are conjoint collateral and closed.

Protoxylem is situated towards the adaxial (upper) surface and protophloem towards the abaxial (lower) surface in the vascular bundle. Leaves are devoid of endodermis and pericycle.

Vascular bundles are surrounded by a bundle sheath. It is made up of thick walled parenchyma.

Druses- crystal of calcium oxalate, star shaped e.g. Nerium.

Cystolith- Crystal of calcium carbonate, like bunch of grapes e.g. Ficus (Banyan, Rubber plant).

Raphides → Crystal of calcium oxalate, Needle shaped e.g. Eichhornia

Notes:- 

In the leaves of C4- plants (eg. sugarcane, maize etc.) bundle sheath is chlorenchymatous.

In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells or motor cells. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.

The size of vascular bundles are dependent on the size of the veins. The veins vary in thickness in thickness in the reticulate venation of the dicot leaves.

The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.

Both upper & lower epidermis of Nerium leaves are multilayered. This is an adaptation to reduce transpiration.

Xerophytes with isobilateral leaves contain palisade tissue on both sides and few amount of spongy tissue is present in between palisade tissue.

Example :- Eucalyptus & Nerium.

1. Epistomatic leaf- stomata are present only on upper surface.

Floating leaves example- lotus (Nelumbium) Victoria regia, Nymphaea.

2. Hypostomatic leaf- stomata are present on lower surface. Mostly dicot leaves.

3. Amphistomatic leaf- stomata are present on both surfaces. Mostly monocot leaves.

4. Astomatic leaf- stomata are absent or non functional. Submerged leaves. Examples- Vallisneria, Hydrilla.

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